Spring (April to June) and autumn (September to October) offer warm, pleasant weather and slightly thinner crowds. July and August are hot and very busy around the Duomo and Uffizi, while winter is cool and much quieter, with occasional rain and shorter museum queues.
Pickpocketing is common in the tight crowds around the Duomo, Ponte Vecchio, the central market and on busy buses. Keep bags zipped and in front of you, carry only what you need, and stay alert when someone bumps or distracts you.
Summer heat can be intense with little shade on the squares and long museum queues. Carry water, refill at the city's public fountains, wear a hat, and plan indoor sights for the hottest afternoon hours.