Potency testing is an important part of the evaluation of cellular therapy products. Potency assays are quantitative measures of a product-specific biologic activity that is linked to a relevant biologic property and, ideally, a product's in vivo mechanism of action. Both in vivo and in vitro assays can be used for potency testing. Since there is often a limited period of time between the completion of production and the release from the laboratory for administration to the patient, in vitro assays such as flow cytometry, ELISA, and cytotoxicity are typically used. Better potency assays are needed to assess the complex and multiple functions of cellular therapy products, some of which are not well understood. Gene expression profiling using microarray technology has been widely and effectively used to assess changes of cells in response to stimuli and to classify cancers. Preliminary studies have shown that the expression of non-coding microRNA (miRNA), which plays an important role in cellular development, differentiation, metabolism, and signal transduction, can distinguish between different types of stem cells and leukocytes. Both gene and miRNA expression profiling have the potential to be important tools for testing the potency of cellular therapies. Potency testing, the complexities associated with potency testing of cellular therapies, and the potential role of gene and miRNA expression microarrays in potency testing of cellular therapies are discussed.