Discovering, developing, and validating biomarkers has become an integral part of novel drug development and drug-targeted flow cytometric assessments have become increasingly important. Flow cytometry can add valuable information during the clinical phases of therapeutic evaluation, but like other technologies, there are many considerations and potential complications that must be considered. The design and level of validation for flow cytometric biomarker assessments is critical to the intended use of the results. While some assays are designed to yield exploratory information, others are validated for patient inclusion criteria or as drug targeted assessments and can yield predictive biomarkers that could potentially lead to a companion diagnostic. There are important considerations, and challenges, regarding steps required to meet all criteria for assay development and validation. Essential to this process is the exchange of specific information between sponsor and provider related to the complexity of the assay, regulatory requirements, intended use of the data, and global deployment. Equally important are the critical steps that must be taken to ensure data consistency between different flow instruments and across all testing sites. This can be accomplished by strictly following global SOPs and having standardized/harmonized processes in place for instrument calibration, reagent qualification and data analysis. This presentation will address all of the key elements related to flow assay design and successful implementation.