Immunoreagents, such antibodies, must often be functionalized before being used for a specific application, and this modification can strongly affect their biophysical characteristics. In contrast, recombinant antibody fragments can be engineered with elevated precision at selected residues to obtain customized reagents. Several alternative strategies can be used that exploit different principles and require different effort to reach this objective. Direct labeling can be obtained by fusing the antibody fragment to a fluorescent protein; otherwise, tag sequences can be appended to the binders and then these are modified by chemical or enzymatic reactions. In this contribution, a model system will be illustrated and compared with other options to illustrate what factors should be considered to choose the functionalization strategy.