Kinases are important therapeutic targets. Potent in vitro inhibitors are readily produced, but these compounds are not always potent in cells. Because cell potency is crucial for efficacy, cell-based assays are important for successful inhibitor development. In this review kinase cell assays are divided into four types. Strengths and weaknesses of each and the value of combining types are discussed. With the current growth of kinase-based therapeutics, review of this field is important and timely.