In experimental economics, where subjects participate in different sessions, observations across subjects of a given session might exhibit more correlation than observations across subjects in different sessions. The problem of session effects is related to similar problems in many experimental and non-experimental fields. This paper attempts to clarify what the issues are and proposes a set of practical tests to identify the problem as well as ways to test for treatment effects in the presence of session-effects. Simulations are used to assess how these tests perform given the relatively small samples typical of experimental data sets.