From its origins in the 16thcentury, microscopy has allowed the cell, as the basic unit of eukaryotic life and disease, to be identified and analyzed. Today, quantitative cytometric technologies, either microscope based or flow cytometric, are the most powerful tools to analyze the proliferation, physiology and differentiation of cells generally, and are particularly useful in immunopathology. In combination with monoclonal antibodies (which recognize specific gene products) conjugated to sensitive fluorescent dyes, cell types can be identified according to the genes they express. They can also be isolated using either fluorescence-activated cell sorting (FACS) or magnetic cell sorting (MACS). In the past 20 years, immunofluorescence-based cytometry and cell sorting have become 'state of the art' technologies, mostly serving to identify subsets of lymphocytes and systemic changes in the immune system. Although it is certainly of value for diagnosis and analysis of immunopathology, cytometry did have one major limitation; except in a few experimental situations, it was not possible to focus analysis on those lymphocytes that specifically recognize the relevant antigens in a normal or pathological immune reaction. This drawback has recently been overcome both for B and T lymphocytes, using antigen to identify the cells. Today, a number of exciting new technologies make it possible to analyze and isolate specifically those lymphocytes that are directly involved in the immune reaction to given antigens. These advances will spur research in arthritis considerably.