The Autoridad Metropolitana de Autobuses (AMA) operates buses that thread through San Juan, running in exclusive lanes on major thoroughfares and stopping at signs marked “Parada.” Destinations are indicated above the windshield. Bus B-21 will take you from Condado to Old San Juan. Bus A-5 runs from San Juan through Santurce and the beach area of Isla Verde. Note that bus service has been reduced significantly due to the island's financial crisis, with route elimination and reduction in service introduced in phases beginning in late 2015. Service starts at around 5 am and generally continues until 9 pm, but buses are not viewed by visitors as an efficient or easy way of getting around San Juan. Fares are 75¢ and are paid in exact change upon entering the bus. Most buses are air-conditioned and have wheelchair lifts and lockdowns.
There's no bus system covering the rest of the island. If you do not have a rental car, your best bet is to travel by públicos, which are usually shared 17-passenger vans. They have license plates ending in "P" or "PD," and they scoot to towns throughout the island, stopping at a terminal in each community's main plaza. They operate primarily during the day; routes and fares are fixed by the Public Service Commission, but schedules aren't set, so you have to call ahead.
Autoridad Metropolitana de Autobuses. 787/294–0500; dtop.gov.pr/transporte_urbano.