Cell-based methods are known for variability. Living systems introduce complexity not present in simple in vitro methods.  Responses to small changes in reagents, in analyst technique, and in the environment can have a major impact on cell-based systems. Cells are the primary critical reagent in any cell-based system, whether in production or in a test method. As in any method, control of the critical reagent is essential for reducing variability. Proper handling of cells and the implementation of controls that are an integral part of the method can reduce variability in cell-based methods.  Attendees of this webinar will be introduced to concepts designed to assist with developing cell-based methods focused on a reduction in assay failures. We will address understanding sources of variability and examine the critical characteristics of cell-based assays. Specifically, the webinar addresses characterization of the cell line, cell maintenance, culturing cell prior to assay set-up as well as for assay set-up. Although the cells are the primary critical reagent, this webinar will also review other critical reagents important in cell culture and cell-based methods. To assist with monitoring we will discuss intermediate performance controls assessed by control charts. Use of alert and action limits with control charts will also be addressed.  Finally, writing of the methods and training of analysts to decrease variability will be addressed.