Small-molecule-protein conjugates are often required to act as immunogeneic complexes in the production of both monoclonal and polyclonal antibodies against small antigens. When antibodies have been obtained, they (and/or the small antigens) need to be labeled to facilitate their use in diagnostic assays. It is often impossible or extremely expensive to obtain the required conjugates. This chapter therefore discusses the common procedures used to couple small molecules to proteins and the analysis of the resulting conjugates. Practical guidance is given on the coupling of small molecule carboxyl, hydroxyl, and amine residues to amine and sulfhydryl residues on proteins using linkage techniques in which the author has extensive experience. Although a comprehensive list and analysis of every available linker is not given, the practical advice should enable the reader to use any commercially available linker productively to its optimum potential.