Trains are Italy's default transport. But buses win in 3 situations: budget (FlixBus Rome→Naples from €5 vs €12+ train), routes without rail (Florence→Siena by bus is faster than the indirect train), and hill towns (buses are the ONLY public transport to San Gimignano, Cortona, Montepulciano, most of Umbria).
FlixBus connects all major Italian cities + 30+ European countries. Prices: Rome→Naples from €5. Rome→Florence from €7. Milan→Venice from €8. Rome→Milan from €12. Book on flixbus.it or the app. WiFi, toilets, power outlets on board. When FlixBus wins: When you're flexible on time (buses take 30-60 min longer than Frecciarossa but cost 50-80% less). When trains win: When speed matters (Rome→Milan: 2h50 by Frecciarossa vs 6-7h by bus).
Tuscany (SITA/Tiemme): Florence→Siena (1h15, €8 — FASTER than the train which goes via Empoli). Florence→San Gimignano (1h30, €7 — no train to San Gimignano). Umbria (Sulga/Busitalia): Perugia→Assisi (30 min), Spoleto→Norcia. Campania (SITA Sud): Amalfi Coast bus (Sorrento→Positano→Amalfi, €2.50 — the famous cliff road, stand on the RIGHT side for sea views). Sicily (AST/Interbus): Limited but essential for Agrigento→Piazza Armerina, Catania→Taormina.
Full airport transfer guide → Rome Fiumicino: SIT/Terravision/TAM bus to Termini, €6, 50 min. Milan Malpensa: Terravision/Autostradale, €8-10, 60 min. Venice Marco Polo: ATVO/ACTV, €8-10, 20 min to Piazzale Roma.