This introduction outlines general strategies for labeling proteins, with an emphasis on methods that are used primarily for labeling antibodies. It covers the specific site of modification, cross-linker options, types of labels, and postlabeling cleanup methodology, along with the advantages and disadvantages of each method. In general, polyclonal antibodies are more versatile and resistant to activity loss than are monoclonal antibodies. Greater care must be taken when labeling monoclonal antibodies to ensure a quality conjugate. The methods outlined here can be adapted for a variety of labels including multiple labels on the same immunoglobulin. The most important consideration when undertaking an antibody labeling experiment is to maintain the activity of the antibody. This is an empirical process and will often require additional experiments to optimize the label of a particular antibody. When successful, these reagents are very useful and adaptable biomolecules. This introduction provides the reader with methods and options for producing a variety of labeled immunological tools.