They tend to be slow and crowded, especially during the city's morning and evening commuter rushes.
Better yet, use the Istanbulkart transit pass.
Besides the İETT buses operated by the city government, Istanbul has private buses owned and operated by private individuals and companies in accordance with city regulations.
Called Özel Halk Otobüsü ("Private Peoples' Bus"), these buses run on some of the busier routes, charging the same fares as city buses, but may allow you to pay your fare in cash (as well as by token or transit pass) on the bus. Otherwise, there's little difference.
If you're packed into the mid-section of the bus when your stop comes, say İnecek var! (een-eh-JEHK vahr, "Someone wants to get out!") so the driver will stop and wait for you to make your way out.
Istanbul Metropolitan Municipality Presidency
Şahkulu Mah. Erkan-ı Harp Sok. No:2, Tünel
34420 Beyoğlu, Istanbul
İETT Tel: +90 (212) 444 1871
Directorate-General Tel: +90 (212) 372 2222 (pbx)
Fax: +90 (212) 243 0883
—by Tom Brosnahan