Koh Chang is reached by ferry from the mainland. The usual route is to get to Trat, the nearest town on the coast, and then take a songthaew to one of the ferry piers at Laem Ngop. Ferries run frequently during the day and the crossing takes about 30 minutes.
From Bangkok, air-conditioned buses leave from the Eastern Bus Terminal (Ekamai) on Sukhumvit Road roughly every hour. The ride to Trat takes five to six hours. Minibus services also run from Bangkok and other tourist towns, often with the ferry included in the price. From Trat, it is a short ride to the pier.
There is also a small airport in Trat with flights from Bangkok, which cuts the journey down considerably if you are willing to pay for it.