Often cheaper than trains, sometimes substantially so, long-distance buses also tend to be slower and less comfortable. However in Portugal, Greece and Turkey, buses are often a better option than trains.
Europe’s biggest organisation of international buses operates under the name Eurolines (www.eurolines.com), comprised of various national companies. A Eurolines Pass (www.eurolines.com/en/eurolines-pass) is offered for extensive travel, allowing passengers to visit a choice of 49 cities across Europe over 15 or 30 days. In the high season (mid-June to mid-September), the pass costs €315/405 for those aged under 26, or €375/490 for those 26 and over. It’s cheaper in other periods.
Busabout (www.busabout.com) offers a 'hop-on, hop-off' service around Europe, stopping at major cities. Buses are often oversubscribed, so book each sector to avoid being stranded. It departs every two days from early May to early October.
Domestic buses provide a viable alternative to trains in most countries. Again, they are usually slightly cheaper and somewhat slower. Buses are generally best for short hops, such as getting around cities and reaching remote villages, and they are often the only option in mountainous regions.
Reservations are rarely necessary. On many city buses you usually buy your ticket in advance from a kiosk or machine and validate it on entering the bus.