Retrospective analysis of marketed drugs has revealed that very low prevalence of P-glycoprotein (P-gp) efflux and moderate to high passive permeability are two key properties that distinguish drugs which engage targets in the central nervous system (CNS) from those that act predominantly in the periphery. Quantification of the efflux properties that differentiate CNS drugs has been determined using in vitro assays for human and rodent P-gp,1,2 and revealed in vivo by comparing brain drug exposures in P-gp (mdr1a/b(−/−) ) knockout mice against those observed in wild type mice.3 Recognition by P-gp not only dramatically affects the distribution properties of compounds into bodily compartments including the CNS, testis, and placenta but also confers resistance to certain cancer chemotherapeutics. This perspective is focused on structural modifications and strategies that can be applied during compound optimization in order to modulate the P-gp efflux properties of small molecules with a particular emphasis on implications for CNS penetration. The topic was last reviewed in 2006 by Raub4 who noted at the time a surprising paucity of published examples where P-gp efflux had been purposefully circumvented. Greater awareness of the implications of efflux coupled with advances in the quality and throughput of in vitro permeability and efflux assays,5 greater access to genetic mouse models that lack expression of P-gp,6,7 and the development of tool compounds that inhibit efflux transporters8 has led to an improved understanding of how structural properties influence efflux recognition. However, the precise molecular interactions that confer P-gp efflux remain relatively poorly defined. The promiscuous nature of P-gp renders the rational circumvention of efflux, while simultaneously optimizing compounds for efficacy, safety, and pharmacokinetic properties, an extremely challenging undertaking.