Railway companies are free to determine the conditions to use their trains with a pass, so you'll have to ask them. For most mandatory reservations though there are alternatives.

It is also possible to book normal train tickets directly from the train companies or via websites such as www.thetrainline.com. You can compare prices of pass + any reservations with normal tickets.

Your interrail pass is your ticket and doesn't give you the right to a specific seat. For trains that require that all passengers have a reserved seat (mandatory reservation), you must also make a seat reservation. About 90 % of the included trains have no mandatory reservation.

In most countries you can choose to travel with trains that don't require reservations. These trains might take longer time.

Where will you be travelling?