Determining the right sample size for a reliability test is always challenging. If the sample size is too small, not enough failure information can be generated. If the sample is too large, cost and time probably will be wasted. In this presentation, we will discuss several commonly used methods on determining the right sample size for 1) reliability demonstration tests, 2) operational life tests under use condition, 3) accelerated life tests under elevated stresses. The theory behind these methods will be discussed first, and then examples of applying these methods will be provided using commercial software tools.