Istanbul is a large, congested, bustling city. You'll enjoy your visit much more if you know how to get around before you arrive.
Istanbul has two airports, the major Atatürk International Airport near Yesilköy 14 miles west of the city center, reachable by Metro and Sabiha Gökçen Airport on the east side of the Bosphorus.
City bus routes get you to some places you'll want to visit. Buy tickets (less than $1US) before boarding at kiosks (often white fiberglass cubes) labeled "I.E.T.T" and/or "Bilet, or use the prepaid electronic fare system.
You'll find Istanbul's two tram lines useful, even though they're as different as can be. Although the nostalgic 19th-century Istiklal Caddesi tram is more fun, the Eminönü-Zeytinburnu tram is the more useful, and can help you travel between the heart of the tourist district at Sultanahmet Square and the Otogar (bus terminal) and/or Atatürk Airport.
Several lines of Istanbul's Metro system are in operation and two are particularly useful to visitors: starting in Aksaray Square, one goes northeast through the city walls to Otogar. Another line connects Atatürk Airport with Old Istanbul and the Otogar.
Tünel is Istanbul's little 19th-century two-station underground train connecting Karaköy (Galata) and the southwestern end of Istiklal Caddesi. It's convenient and fun.
Suburban commuter trains depart Sirkeci Station, trundle around Seraglio Point and continue along the Sea of Marmara shore stopping at Yesilyurt near Atatürk International Airport. Slightly better suburban trains depart Haydarpasa Station and travel along the northern shore of the Sea of Marmara on the Asian side of the Bosphorus.
Traditional white ferries serve shorter water routes and are the most enjoyable way to get around Istanbul. Special daily Touristic Bosphorus Ferries run from Eminönü's Dock 2 up the Bosphorus almost to the Black Sea several times daily. Both catamarans and ferryboats travel to the Princes Islands near Istanbul in the Sea of Marmara.
Sleek modern passenger catamarans zoom around the city at rush hour, and out to the Princes Islands several times daily. There are even Sea of Marmara routes to Yalova and Bandirma on the sea's southern shore.
Thousands of yellow taxis, most powered by clean-burning natural gas, throng Istanbul's streets. You'll find them useful and not overly expensive.
Dolmus means "filled," which is what the vehicle needs to be before it departs on its customary route. The dolmus is Turkey's shared taxi or minibus. You may find them helpful occasionally in Istanbul.
The best way to get around Istanbul's compact medieval core is on foot. Traffic is often so heavy, and traffic patterns so circuitous, that you can often walk somewhere faster than riding.