Mediation of antinociception via opioid receptors located in the periphery is a viable strategy to produce analgesia without the occurrence of side effects associated with stimulation of opioid receptors located in the central nervous system. Peripheral opioid receptors are particularly important in inflammatory pain states and in the responses to pruritogenic stimuli, and have been implicated in the transmission of visceral pain. Medicinal chemistry approaches to achieve peripheralization of opioid agonists have started with a centrally acting opioid agonist as a template, and introduced features of lipophilicity, hydrophilicity, or combined lipophilicity and hydrophilicity to achieve amphiphilicity. Quaternarization of centrally acting opioid agonists or identification of compounds that serve as substrates for the mdr transporter to achieve transport out of the brain has also been employed. The in vivo assays used to identify peripherally selective compounds have measured a variety of behavioral and pharmacokinetic endpoints, with varying degrees of predictability. This review focuses on a discussion of these methods, as well as a review of those compounds where sufficient data exist to support a claim of peripheralization in vivo.