Santa Maria in Trastevere is one of Rome's oldest parish churches, and what you really need to know is practical: entry to the main nave is usually free, the highlight is the brilliant medieval mosaics in the apse by Pietro Cavallini so allow 20 to 30 minutes to take them in, and the church is still an active place of worship so avoid visiting during Mass if you want to linger or photograph. Dress modestly, be respectful and quiet inside, and know that the small museum/crypt beneath the church sometimes requires a separate ticket. To dodge crowds, go early morning or late afternoon, and plan to linger afterward in the charming piazza for a coffee.