diff --git a/src/api.js b/src/api.js
index f2ad6c7c..e308e58c 100644
--- a/src/api.js
+++ b/src/api.js
@@ -76,6 +76,18 @@ class RestrictedConnection extends common.Connection {
 }
 
 class RippleAPI extends EventEmitter {
+
+  _feeCushion: number;
+  connection: RestrictedConnection;
+
+  // these are exposed only for use by unit tests; they are not part of the API.
+  static _PRIVATE = {
+    validate: common.validate,
+    RangeSet: require('./common/rangeset').RangeSet,
+    ledgerUtils: require('./ledger/utils'),
+    schemaValidator: require('./common/schema-validator')
+  };
+
   constructor(options: APIOptions = {}) {
     common.validate.apiOptions(options)
     super()
@@ -146,14 +158,6 @@ _.assign(RippleAPI.prototype, {
   errors
 })
 
-// these are exposed only for use by unit tests; they are not part of the API
-RippleAPI._PRIVATE = {
-  validate: common.validate,
-  RangeSet: require('./common/rangeset').RangeSet,
-  ledgerUtils: require('./ledger/utils'),
-  schemaValidator: require('./common/schema-validator')
-}
-
 module.exports = {
   RippleAPI
 }
diff --git a/src/common/rangeset.js b/src/common/rangeset.js
index 9c3fa10c..e45e3c2a 100644
--- a/src/common/rangeset.js
+++ b/src/common/rangeset.js
@@ -2,7 +2,6 @@
 'use strict' // eslint-disable-line strict
 const _ = require('lodash')
 const assert = require('assert')
-const ranges = Symbol()
 
 function mergeIntervals(intervals: Array<[number, number]>) {
   const stack = [[-Infinity, -Infinity]]
@@ -19,22 +18,25 @@ function mergeIntervals(intervals: Array<[number, number]>) {
 }
 
 class RangeSet {
+
+  ranges: Array<[number, number]>;
+
   constructor() {
     this.reset()
   }
 
   reset() {
-    this[ranges] = []
+    this.ranges = []
   }
 
   serialize() {
-    return this[ranges].map(range =>
+    return this.ranges.map(range =>
       range[0].toString() + '-' + range[1].toString()).join(',')
   }
 
   addRange(start: number, end: number) {
     assert(start <= end, 'invalid range')
-    this[ranges] = mergeIntervals(this[ranges].concat([[start, end]]))
+    this.ranges = mergeIntervals(this.ranges.concat([[start, end]]))
   }
 
   addValue(value: number) {
@@ -50,7 +52,7 @@ class RangeSet {
   }
 
   containsRange(start: number, end: number) {
-    return _.some(this[ranges], range => range[0] <= start && range[1] >= end)
+    return _.some(this.ranges, range => range[0] <= start && range[1] >= end)
   }
 
   containsValue(value: number) {
diff --git a/src/common/utils.js b/src/common/utils.js
index cf806a4c..1fa0c750 100644
--- a/src/common/utils.js
+++ b/src/common/utils.js
@@ -27,6 +27,7 @@ function toRippledAmount(amount: Amount): RippledAmount {
   if (amount.currency === 'XRP') {
     return xrpToDrops(amount.value)
   }
+  // $FlowFixMe: amount.issuer is not a Amount type property. Safe to remove?
   return {
     currency: amount.currency,
     issuer: amount.counterparty ? amount.counterparty :
diff --git a/src/ledger/orderbook.js b/src/ledger/orderbook.js
index 9d4059a7..2644a4dc 100644
--- a/src/ledger/orderbook.js
+++ b/src/ledger/orderbook.js
@@ -38,7 +38,7 @@ type GetOrderbook = {
 function getBookOffers(connection: Connection, account: string,
     ledgerVersion?: number, limit?: number, takerGets: Issue,
     takerPays: Issue
-): Promise {
+): Promise<Object[]> {
   return connection.request(utils.renameCounterpartyToIssuerInOrder({
     command: 'book_offers',
     taker_gets: takerGets,
diff --git a/src/ledger/orders.js b/src/ledger/orders.js
index bec9edab..4aee1269 100644
--- a/src/ledger/orders.js
+++ b/src/ledger/orders.js
@@ -11,7 +11,7 @@ type GetOrders = Array<Order>
 
 function requestAccountOffers(connection: Connection, address: string,
   ledgerVersion: number, marker: string, limit: number
-): Promise {
+): Promise<Object> {
   return connection.request({
     command: 'account_offers',
     account: address,
diff --git a/src/ledger/parse/transaction.js b/src/ledger/parse/transaction.js
index 8659fade..8539f9b3 100644
--- a/src/ledger/parse/transaction.js
+++ b/src/ledger/parse/transaction.js
@@ -54,7 +54,7 @@ function parseTransaction(tx: Object): Object {
     'feeUpdate': parseFeeUpdate,
     'amendment': parseAmendment
   }
-  const parser = mapping[type]
+  const parser: Function = (mapping: Object)[type]
   assert(parser !== undefined, 'Unrecognized transaction type')
   const specification = parser(tx)
   const outcome = utils.parseOutcome(tx)
diff --git a/src/ledger/parse/utils.js b/src/ledger/parse/utils.js
index 0a2602ac..25f9b8ad 100644
--- a/src/ledger/parse/utils.js
+++ b/src/ledger/parse/utils.js
@@ -6,7 +6,7 @@ const utils = require('../utils')
 const BigNumber = require('bignumber.js')
 const parseAmount = require('./amount')
 
-import type {Amount} from '../common/types.js'
+import type {Amount} from '../../common/types.js'
 
 function adjustQualityForXRP(
   quality: string, takerGetsCurrency: string, takerPaysCurrency: string
@@ -51,7 +51,7 @@ function removeEmptyCounterpartyInOrderbookChanges(orderbookChanges) {
   })
 }
 
-function isPartialPayment(tx) {
+function isPartialPayment(tx: Object) {
   return (tx.Flags & utils.common.txFlags.Payment.PartialPayment) !== 0
 }
 
diff --git a/src/ledger/pathfind.js b/src/ledger/pathfind.js
index 060eef9d..09285fcd 100644
--- a/src/ledger/pathfind.js
+++ b/src/ledger/pathfind.js
@@ -13,14 +13,16 @@ import type {GetPaths, PathFind, RippledPathsResponse, PathFindRequest}
   from './pathfind-types.js'
 
 
-function addParams(request: PathFindRequest, result: RippledPathsResponse) {
+function addParams(request: PathFindRequest, result: RippledPathsResponse
+): RippledPathsResponse {
   return _.defaults(_.assign({}, result, {
     source_account: request.source_account,
     source_currencies: request.source_currencies
   }), {destination_amount: request.destination_amount})
 }
 
-function requestPathFind(connection: Connection, pathfind: PathFind): Promise {
+function requestPathFind(connection: Connection, pathfind: PathFind
+): Promise<RippledPathsResponse> {
   const destinationAmount = _.assign({value: -1}, pathfind.destination.amount)
   const request: PathFindRequest = {
     command: 'ripple_path_find',
@@ -76,7 +78,7 @@ function isRippledIOUAmount(amount: RippledAmount) {
 
 function conditionallyAddDirectXRPPath(connection: Connection, address: string,
   paths: RippledPathsResponse
-): Promise {
+): Promise<RippledPathsResponse> {
   if (isRippledIOUAmount(paths.destination_amount)
       || !_.includes(paths.destination_currencies, 'XRP')) {
     return Promise.resolve(paths)
diff --git a/src/ledger/utils.js b/src/ledger/utils.js
index 25849d41..6f6323a7 100644
--- a/src/ledger/utils.js
+++ b/src/ledger/utils.js
@@ -22,7 +22,7 @@ function clamp(value: number, min: number, max: number): number {
 
 function getXRPBalance(connection: Connection, address: string,
   ledgerVersion?: number
-): Promise<number> {
+): Promise<string> {
   const request = {
     command: 'account_info',
     account: address,
@@ -34,7 +34,7 @@ function getXRPBalance(connection: Connection, address: string,
 
 // If the marker is omitted from a response, you have reached the end
 function getRecursiveRecur(getter: Getter, marker?: string, limit: number
-): Promise {
+): Promise<Array<any>> {
   return getter(marker, limit).then(data => {
     const remaining = limit - data.results.length
     if (remaining > 0 && data.marker !== undefined) {
@@ -46,7 +46,7 @@ function getRecursiveRecur(getter: Getter, marker?: string, limit: number
   })
 }
 
-function getRecursive(getter: Getter, limit?: number): Promise {
+function getRecursive(getter: Getter, limit?: number): Promise<Array<any>> {
   return getRecursiveRecur(getter, undefined, limit || Infinity)
 }
 
@@ -112,7 +112,7 @@ function isPendingLedgerVersion(connection: Connection,
 }
 
 function ensureLedgerVersion(options: Object
-): Promise<number> {
+): Promise<Object> {
   if (Boolean(options) && options.ledgerVersion !== undefined &&
     options.ledgerVersion !== null
   ) {
diff --git a/src/transaction/payment-channel-claim.js b/src/transaction/payment-channel-claim.js
index e5ee13af..e970dc15 100644
--- a/src/transaction/payment-channel-claim.js
+++ b/src/transaction/payment-channel-claim.js
@@ -19,7 +19,7 @@ type PaymentChannelClaim = {
 function createPaymentChannelClaimTransaction(account: string,
   claim: PaymentChannelClaim
 ): Object {
-  const txJSON = {
+  const txJSON: Object = {
     Account: account,
     TransactionType: 'PaymentChannelClaim',
     Channel: claim.channel,
diff --git a/src/transaction/payment-channel-fund.js b/src/transaction/payment-channel-fund.js
index fa90997e..68a30522 100644
--- a/src/transaction/payment-channel-fund.js
+++ b/src/transaction/payment-channel-fund.js
@@ -13,7 +13,7 @@ type PaymentChannelFund = {
 function createPaymentChannelFundTransaction(account: string,
   fund: PaymentChannelFund
 ): Object {
-  const txJSON = {
+  const txJSON: Object = {
     Account: account,
     TransactionType: 'PaymentChannelFund',
     Channel: fund.channel,
diff --git a/src/transaction/settings-types.js b/src/transaction/settings-types.js
deleted file mode 100644
index 5d57b30f..00000000
--- a/src/transaction/settings-types.js
+++ /dev/null
@@ -1,53 +0,0 @@
-/* @flow */
-'use strict' // eslint-disable-line strict
-
-type SettingPasswordSpent = {
-  passwordSpent?: boolean,
-}
-type SettingRequireDestinationTag = {
-  requireDestinationTag?: boolean,
-}
-type SettingRequireAuthorization = {
-  requireAuthorization?: boolean,
-}
-type SettingDisallowIncomingXRP = {
-  disallowIncomingXRP?: boolean,
-}
-type SettingDisableMasterKey = {
-  disableMasterKey?: boolean,
-}
-type SettingEnableTransactionIDTracking = {
-  enableTransactionIDTracking?: boolean,
-}
-type SettingNoFreeze = {
-  noFreeze?: boolean,
-}
-type SettingGlobalFreeze = {
-  globalFreeze?: boolean,
-}
-type SettingDefaultRipple = {
-  defaultRipple?: boolean,
-}
-type SettingEmailHash = {
-  emailHash?: ?string,
-}
-type SettingMessageKey = {
-  messageKey?: string,
-}
-type SettingDomain = {
-  domain?: string,
-}
-type SettingTransferRate = {
-  transferRate?: ?number,
-}
-type SettingRegularKey = {
-  regularKey?: string
-}
-
-export type Settings = SettingRegularKey |
-  SettingTransferRate | SettingDomain | SettingMessageKey |
-  SettingEmailHash | SettingDefaultRipple |
-  SettingGlobalFreeze | SettingNoFreeze | SettingEnableTransactionIDTracking |
-  SettingDisableMasterKey | SettingDisallowIncomingXRP |
-  SettingRequireAuthorization | SettingRequireDestinationTag |
-  SettingPasswordSpent
diff --git a/src/transaction/settings.js b/src/transaction/settings.js
index 02aebf5e..4aa927ec 100644
--- a/src/transaction/settings.js
+++ b/src/transaction/settings.js
@@ -8,17 +8,36 @@ const validate = utils.common.validate
 const AccountFlagIndices = utils.common.constants.AccountFlagIndices
 const AccountFields = utils.common.constants.AccountFields
 import type {Instructions, Prepare} from './types.js'
-import type {Settings} from './settings-types.js'
+
+type Settings = {
+  passwordSpent?: boolean,
+  requireDestinationTag?: boolean,
+  requireAuthorization?: boolean,
+  disallowIncomingXRP?: boolean,
+  disableMasterKey?: boolean,
+  enableTransactionIDTracking?: boolean,
+  noFreeze?: boolean,
+  globalFreeze?: boolean,
+  defaultRipple?: boolean,
+  emailHash?: ?string,
+  messageKey?: string,
+  domain?: string,
+  transferRate?: ?number,
+  regularKey?: string,
+  signers?: {
+    threshold?: number,
+    weights: {address: string, weight: number}[],
+  },
+}
 
 // Emptry string passed to setting will clear it
 const CLEAR_SETTING = null
 
-
 function setTransactionFlags(txJSON: Object, values: Settings) {
   const keys = Object.keys(values)
   assert(keys.length === 1, 'ERROR: can only set one setting per transaction')
   const flagName = keys[0]
-  const value = values[flagName]
+  const value = (values: Object)[flagName]
   const index = AccountFlagIndices[flagName]
   if (index !== undefined) {
     if (value) {