If you plan to re-enter Thailand on your existing visa. For example - if you enter Thailand on a single entry tourist visa and want to make a side trip to Cambodia and want to re-enter Thailand on your tourist visa, you would have to obtain a re-entry permit before leaving Thailand.

There are two types of re-entry permits -
1) Single re-entry permit - As the name suggests, this permit allows you to re-enter Thailand only once. The duration of the permit is the same as your existing Thai visa. The fee for this permit is 1000 THB.
2) Multiple re-entry permit - This permit allows you to re-enter Thailand as long as your visa is valid. The fee for this permit is 3800 THB.
Re-entry permits can be obtained at the airport, other immigration checkpoints or at the immigration department office in the city. (Soi Suan Plu in Bangkok).
Discuss Thailand Re-Entry Permit in Thailand Guide Forum