In general, high-speed trains will be ticketless, requiring no validation. Regional trains will require you to validate your ticket before boarding your train for departure. Follow the steps listed {here} to validate your regional tickets.

You should never board a train in Italy without a valid ticket as the penalties are immediate and strictly enforced. If you are caught on board without a ticket, you will be required to pay the full fare plus a "regularization fee" or fine, which typically starts at €50 to €200 depending on the region and the type of train. For high-speed trains (Frecciarossa or Italo), seats are reserved, and boarding without a valid reservation is a serious offense. For regional trains, it is even more complex: as of late 2023, even digital tickets must be "checked in" or validated via the Trenitalia app before the train departs. If you have a physical paper ticket, you must punch it in the green or yellow machines at the station. In very rare cases, if the station ticket office and machines are both broken, you must seek out the conductor immediately upon boarding (before they find you) to declare the issue and purchase a ticket, often still with a small surcharge.