Cellular assays have been used to assess the pre- and post-transplant immune status of recipients throughout the history of clinical transplantation. Initially, bulk culture assays were used; however, more recent refinements in the techniques allow for assaying multiple functions simultaneously including cell subset functional analysis and the response at the single cell level. The intracellular ATP synthesis assay is being incorporated routinely into the clinical testing strategy. Both donor-specific and non-specific testing strategies are being used to evaluate the immune status of recipients allowing for potential points of intervention aimed at decreasing the adverse impact of clinical complications. This review focuses on the current use of cellular testing in clinical transplantation.