Due to their ease of isolation from phage display libraries and their ability to recognize conserved antigens, single-chain Fv (scFv) molecules are rapidly becoming commonplace. However, the monovalent nature of the scFv molecule often dictates, at best, transient interactions with target antigens when molecules with moderate to low affinity are employed. This, along with their rapid elimination from circulation, has limited the utility of scFv molecules for applications in the fields of cancer imaging and therapy. Recently, a number of strategies, including affinity maturation and modification of size and valence, have been evaluated for improving the in vivo efficacy of scFv molecules. In this review, we describe a number of these methods and discuss some of the characteristics that may belong to an optimal antibody-based targeting vehicle.