Flow cytometry has become an essential element of biomedical research in a number of fields beyond immunology, where it originated in the 1970s. Continuous advances in instrumentation have increased the scope of flow cytometry and expanded the types of analyses made possible by this technology. The simultaneous detection of up to 12 fluorescence parameters allows exquisite dissection of cell states, including analysis at the single-cell level of surface expressed proteins, intracellular signaling molecules, and certain enzymatic activities. In addition, sorting at speeds of up to 30,000 cells/sec enables the direct isolation and characterization of even very rare cell populations (present at <0.01 % of total cells). Single cell deposition permits clonal analysis of cell behavior and the application of molecular biology approaches to individual cells of a population, providing a richer and more informative description of gene regulation and responsiveness at the single-cell level. Finally, the recent development of large-object sorting brings the ability to distinguish and analyze complex cell clusters, such as differentiating stem cells or pancreatic islets. Joslin's Flow Cytometry Core Facility has for some time provided Center researchers with an advanced facility for complex, multi-purpose cell analysis and sorting by flow cytometry. The goals of the Core are: 1. To implement and maintain a reliable, multi-functional and cost-efficient Flow Cytometry facility with streamlined operator support to minimize scheduling bottlenecks and machine down-time. 2. To provide education and training to Joslin researchers regarding potential applications of flow cytometry and intricate aspects of sample preparation/handling and of data analysis. 3. To continually update instrumentation in response to new and cutting-edge technology developments, including upgrades to existing instruments, addition of new instrumentation, and establishment of collaborative arrangements to help develop and evaluate new technology.