Immunohistochemistry is a technique to examine the expression and distribution of biomolecules in situ using antibodies labeled with enzymes, such as horseradish peroxidase. Generally, both immunohistochemistry and immunofluorescence are termed as immunostaining, because localized biomolecules in situ are detected with antigen-antibody immunoreactions producing color signals. Immunostaining techniques are divided into 3 types: direct, indirect, and amplification methods (peroxidase-antiperoxidase, avidin-biotin complex, and polymer). Because each method has advantages and disadvantages, the method should be selected according to the biological purpose. In this technical note, we describe mechanisms and procedures for immunostaining, including immunohistochemistry and immunofluorescence.