There are many elements in plasma that can act as surrogate markers of the physiological well-being of a mouse, thus making the collection of blood and plasma a general technique with many applications in mouse phenotyping. For example, the presence of certain enzymes in plasma can serve as markers of tissue toxicity (AST, ALT) and general function, and the more sophisticated lipid and lipoprotein profile tests (cholesterol, LDL) can point to dyslipidemias. As many of the tests available to measure these parameters have been adapted to automated systems in a high-throughput fashion, they have become part of the first line of screening protocols in mouse phenotyping. In this section, general techniques associated with collection and processing of blood are described