Antibodies are labeled for the detection of antigens. Antigens can be detected in two ways: direct method and indirect method (see Figure 5.10 and Figure 5.11). In the direct method, the antigen is detected with the labeled primary antibodies. Therefore, it is necessary to purify antibodies prior to labeling. In the indirect method of detection, labeling is performed in the secondary antibodies that bind to the unlabeled primary antibodies. Labeled secondary antibodies are commercially available. The choice of the direct or the indirect method depends on several factors. Both methods have some advantages and disadvantages (Table 6.1), but the indirect method is used for most applications. This chapter describes several procedures of how antibodies can be labeled and the labeled antibodies can be detected. Some immunochemical techniques are also described.