Many factors can affect the successful implementation and validity of intervention studies. A primary purpose of feasibility and pilot studies is to assess the potential for successful implementation of the proposed main intervention studies and to reduce threats to the validity of these studies. This article describes a typology to guide the aims of feasibility and pilot studies designed to support the development of randomized controlled trials and provides an example of the studies underlying the development of one rehabilitation trial. The purpose of most feasibility and pilot studies should be to describe information and evidence related to the successful implementation and validity of a planned main trial. Null hypothesis significance testing is not appropriate for these studies unless the sample size is properly powered. The primary tests of the intervention effectiveness hypotheses should occur in the main study, not in the studies that are serving as feasibility or pilot studies.