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diff --git a/README.md b/README.md
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+# Market Watch
+
+A console-based application to watch stock prices and currency exchange rates.
+
+## Setup
+
+```
+pip install -r requirements.txt
+```
+
+### API Endpoints
+
+The application queries the following APIs. You need to register an account on
+Rapid API and subscribe to them. The application is written to try to stay under
+the request limit of the free tiers, although that will also depend on how many
+stocks and currencies you decide to query. For a small set, the free tier of the
+APIs should be sufficient. Otherwise, consider subscribing to the paid tiers.
+
+- Stock: https://rapidapi.com/amansharma2910/api/realstonks
+- Currency: https://rapidapi.com/juhestudio-juhestudio-default/api/exchange-rate-api1
+
+Once you have created an account and subscribed, define the environment variable
+`RAPIDAPI_KEY` with your API key prior to running the application. The
+application queries this variable to determine your API key.
+
+### Stocks and Currency
+
+The application state is persisted in the file `state.txt`. To define which
+stocks to query, add/remove the necessary lines to this file.
+
+The syntax of a stock line is:
+
+```
+sticker price change change%
+```
+
+The syntax of a currency exchange is:
+
+```
+from/to rate
+```
+
+For stocks, you can leave all fields but the sticker to 0. For exchange rates,
+you can leave the rate to 0.
+
+See examples in the provided state file.
diff --git a/market.py b/market.py
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+#!/usr/bin/python
+import argparse
+from collections import namedtuple
+import requests
+import os
+import sys
+import time
+
+from textual.app import App, ComposeResult
+from textual.widgets import DataTable
+
+# Rapid API key must be set in the environment.
+RAPIDAPI_KEY = os.environ.get('RAPIDAPI_KEY')
+
+# The file where the application's state is persistent.
+STATE_FILE = "state.txt"
+
+# API endpoint.
+Endpoint = namedtuple('Endpoint', ['url', 'headers', 'update_delay'])
+
+# API endpoints.
+ENDPOINTS = {
+    'stock': Endpoint(url="https://realstonks.p.rapidapi.com/", headers={
+        "X-RapidAPI-Key": f"{RAPIDAPI_KEY}",
+        "X-RapidAPI-Host": "realstonks.p.rapidapi.com"
+    }, update_delay=5 * 60),  # 5 minutes
+    'currency': Endpoint(
+        url="https://exchange-rate-api1.p.rapidapi.com/convert", headers={
+            "X-RapidAPI-Key": f"{RAPIDAPI_KEY}",
+            "X-RapidAPI-Host": "realstonks.p.rapidapi.com"
+        }, update_delay=60 * 60)  # 1 hour
+}
+
+# Application state.
+State = namedtuple('State', ['stocks', 'exchanges'])
+
+# Stock quote.
+Stock = namedtuple('Stock',
+                   ['sticker', 'price', 'change_point', 'change_percent'])
+
+# Exchange rate.
+Exchange = namedtuple('Exchange', ['source', 'target', 'rate'])
+
+
+def get_stock(stickers: list[str]) -> list[Stock]:
+    """Query the stock prices for the given stickers.
+
+    The result may not have prices for all the input stickers if some of the
+    queries fail. This function attempts to get as many prices as possible such
+    that failure in a query does not preclude other stocks from being queried.
+    """
+    # This API does not allow querying multiple stickers in a single request.
+    # Free tier: 100,000 requests/month.
+    #
+    # Make sure that a request failure does not preclude from getting other
+    # stocks.
+    #
+    # Example response:
+    # {
+    #   "price": 466.4,
+    #   "change_point": 7.4,
+    #   "change_percentage": 1.61,
+    #   "total_vol": "11.29M"
+    # }
+    stocks = []
+    for sticker in stickers:
+        try:
+            endpoint = ENDPOINTS['stock']
+            response = requests.get(f"{endpoint.url}{sticker}",
+                                    headers=endpoint.headers).json()
+            stocks.append(
+                Stock(sticker, float(response['price']),
+                      float(response['change_point']),
+                      float(response['change_percentage'])))
+        except Exception as e:
+            print(e)
+    return stocks
+
+
+def get_exchange_rate(source: str, target: str) -> float:
+    """Get the exchange rate between two currencies. Return 0 on failure."""
+    # Free tier:
+    #
+    # Example response:
+    # {
+    #   "code": "0",
+    #   "msg": "success",
+    #   "convert_result": {
+    #     "base": "USD",
+    #     "target": "EUR",
+    #     "rate": 0.9063
+    #   },
+    #   "time_update": {
+    #     "time_unix": 1690556940,
+    #     "time_utc": "2023-07-28T08:09:00Z",
+    #     "time_zone": "America/Los_Angeles"
+    #   }
+    # }
+    try:
+        query = {"base": source, "target": target}
+        endpoint = ENDPOINTS['currency']
+        response = requests.get(endpoint.url, headers=endpoint.headers,
+                                params=query).json()
+        return float(response['convert_result']['rate'])
+    except Exception as e:
+        print(e)
+    return 0.0
+
+
+def update_stocks(state: State) -> State:
+    stickers = [stock.sticker for stock in state.stocks]
+    updated_stocks = get_stock(stickers)
+    # Note that updated_stocks may not have all the stocks in the input.
+    updated_stocks_stickers = [stock.sticker for stock in updated_stocks]
+    missing_stocks = [stock for stock in state.stocks if
+                      stock.sticker not in updated_stocks_stickers]
+    stocks = updated_stocks + missing_stocks
+    return State(stocks, state.exchanges)
+
+
+def update_exchanges(state: State) -> State:
+    exchanges = []
+    for exchange in state.exchanges:
+        rate = get_exchange_rate(exchange.source, exchange.target)
+        if rate != 0:
+            exchanges.append(Exchange(exchange.source, exchange.target, rate))
+        else:
+            exchanges.append(exchange)
+    return State(state.stocks, exchanges)
+
+
+def format_delta(stock: Stock, percent: bool = False) -> str:
+    sign = "+" if stock.change_point >= 0 else "-"
+    change = f"{sign}{abs(stock.change_point)}{'%' if percent else ''}"
+    return change
+
+
+def format_exchange_name(exchange: Exchange) -> str:
+    return f"{exchange.source}/{exchange.target}"
+
+
+def load_state(filepath: str) -> State:
+    stocks = []
+    exchanges = []
+
+    lines = []
+    with open(filepath, 'r') as file:
+        lines = file.readlines()
+
+    for line in lines:
+        values = line.split(' ')
+        key = values[0]
+        if '/' in key:
+            source, target = key.split('/')
+            rate = float(values[1])
+            exchanges.append(Exchange(source, target, rate))
+        else:
+            sticker = key
+            price = float(values[1])
+            change_point = float(values[2])
+            change_percent = float(values[3])
+            stocks.append(
+                Stock(sticker, price, change_point, change_percent))
+
+    return State(stocks, exchanges)
+
+
+def save_state(state: State, filepath: str):
+    with open(filepath, 'w') as file:
+        for stock in state.stocks:
+            values = [str(x) for x in list(stock)]
+            file.write(f"{' '.join(values)}\n")
+
+        for exchange in state.exchanges:
+            file.write(f"{format_exchange_name(exchange)} {exchange.rate}\n")
+
+
+class Updater:
+    def __init__(self, update, delay):
+        self.update = update
+        self.delay = delay
+        self.last_update_time = 0
+
+
+def update_stub(msg: str, state: State) -> State:
+    print(msg)
+    return state
+
+
+def make_updaters(use_stubs: bool) -> list[Updater]:
+    updaters = []
+    if use_stubs:
+        updaters = [
+            Updater(lambda s: update_stub("Update stocks", s), 1),
+            Updater(lambda s: update_stub("Update exchange", s), 5)
+        ]
+    else:
+        updaters = [
+            Updater(update_stocks, ENDPOINTS['stock'].update_delay),
+            Updater(update_exchanges, ENDPOINTS['currency'].update_delay)
+        ]
+    return updaters
+
+
+def update_state(t: float, updaters: list[Updater], state: State) -> State:
+    for updater in updaters:
+        if t - updater.last_update_time >= updater.delay:
+            state = updater.update(state)
+            updater.last_update_time = t
+    return state
+
+
+class MarketApp(App):
+    TITLE = "Market Watch"
+
+    def __init__(self, updaters: list[Updater]):
+        super().__init__()
+        self.state = None
+        self.table = None
+        self.updaters = updaters
+        self.min_update_delay = min([updater.delay for updater in updaters])
+
+    def render(self):
+        assert self.state is not None
+        assert self.table is not None
+
+        #  Stock/ex | Price | Change
+        #   xyz     |  xxx  |  xxx
+        #  usd/eur  |  xxx  | <empty>
+        table = self.table
+        table.clear(columns=True)
+        table.add_columns("Stock", "Price($)", "Change($)", "%")
+        for stock in self.state.stocks:
+            table.add_row(stock.sticker, stock.price, format_delta(stock),
+                          format_delta(stock, percent=True))
+        for exchange in self.state.exchanges:
+            table.add_row(format_exchange_name(exchange), exchange.rate, "", "")
+
+    def compose(self) -> ComposeResult:
+        self.state = load_state(STATE_FILE)
+
+        table = DataTable()
+        table.show_cursor = False
+        self.table = table
+        yield table
+
+        self.render()
+
+        self.update()
+        self.set_interval(self.min_update_delay, self.update)
+
+    def update(self) -> None:
+        t = time.time()
+        self.state = update_state(t, self.updaters, self.state)
+        self.render()
+        save_state(self.state, STATE_FILE)
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--stub", action='store_true',
+                        help="Use stub update functions")
+    args = parser.parse_args()
+
+    updaters = make_updaters(args.stub)
+
+    app = MarketApp(updaters)
+    app.run()
+
+
+if __name__ == '__main__':
+    sys.exit(main())
diff --git a/requirements.txt b/requirements.txt
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+textual
diff --git a/state.txt b/state.txt
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+NVDA 445.8 -1.0 -0.22
+AMD 115.73 -0.09 -0.08
+USD/EUR 0.9