There are two national bus companies in Cuba: Viazul and Astro.

Viazul is the most luxurious and dependable one as it is international tourist oriented.
Astro is more axed on Cuban and budget travellers and therefore uses outdated equipment and is not as reliable. Cuban tourist authorities push tourists towards Viazul and with a deadline to meet that is your best bet.

More adventurous travellers can try Astro.

Note that the bus is a good way to get around in Cuba. You get to see some of the countryside and experience more of the country than on a plane or train.

Viazul:

Avenida de los Libertadores, esquina a Yarayo
Tel:+53 (226) 2 8484

The site offers online reservations.

• Viazul website with routes

• Havana - Santiago de Cuba route

• Varadero - Santiago de Cuba route

• Trinidad - Santiago de Cuba route

• Santiago de Cuba - Baracoa route 

Astro:

Office:

Ave. De los Libertadores.

Terminal de Ómnibus ASTRO.
Tel: +53 (22) 62 3050, 654036

• Astro Routes

• Time schedule

• Havana - Santiago de Cuba Prices

Note: the links to the Astro sites are very unreliable. Astro is meant to be a company for Cubans while Viazul (linked to Astro) is the "tourist" part of bus transportation in Cuba.