To decide whether experience teaches that small proteins and constrained peptides having high affinity for molecular targets can be engineered to have suitable pharmacokinetics for imaging. Phage display, a molecular diversity technology, allows selection of polypeptides having high affinity and specificity for almost any target. These polypeptides can be modified in ways that improve pharmacokinetics with acceptable impact on binding. Often, relatively few changes confers pharmacokinetics suitable for imaging on polypeptides selected for affinity and specificity to a target and for stability. It is likely that few variants of phage-selected proteins and constrained peptides will need to be tested to obtain a useful imaging agent.