Book excerpt: There hasn’t been a better time to be a drug developer for immune-based therapies than the past couple of decades. We have seen an explosion in immune-based therapies for cancer, autoimmune and infectious diseases, metabolic diseases and diseases and disorders of the nervous system. The modalities of these immune-based therapies span small molecules, biologics, and gene and cell therapeutic approaches. Significant advances have been made in optimizing drug design for its specificity for the target, characterizing the mode of action in in vitro assays, and ensuring safety and manufacturability. However, an area of challenge that remains is identifying animal models for evaluating efficacy and pharmacokinetics/pharmacodynamics relationship that are predictive of drug effects in humans. Discussion on this topic is warranted as examples of failures of translation from animal models to humans provide us an opportunity to learn more about human biology.