A variety of complementary modalities are used for the diagnosis and classification of lymphoma and leukemia. Cytology and/or histology are frequently the first steps, but these methods alone often do not provide prognostic information or point to clear treatment choices. Clonality testing can be used to establish if a malignancy is present when other diagnostics are ambiguous. Flow cytometry is now commonly used to subclassify these diseases into clinically relevant groups with different outcomes and treatment choices. Flow cytometry is also used to measure the proliferation of neoplastic cells, which provides prognostic information in some tumors, such as B cell chronic lymphocytic leukemia. Other methods, such as gene expression and mutation analysis, are just on the horizon and will almost certainly be in common use in the next 3–5 years. The goal of this session will be to describe what testing methods are most useful for achieving a diagnosis and prognosis, the principles underlying those methods, and the technical aspects of collecting samples.