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Enrichment designs are being increasingly used in fields such as chronic pain research because they may better reflect routine clinical practice than other study designs. Researchers use a variety of methods to perform the first stage of an enrichment design trial. This chapter examines the varied strategies involved in developing a trial using an enrichment design, the advantages and disadvantages of this method and considers the issues when planning a study using enrichment strategies. Issues related to washout periods were considered in the trial using the enrichment design to examine the use of gabapentin for treatment of postherpetic neuralgia (PHN). The distinction between enrichment by response and enrichment by expected mechanism of action is significant. An enrichment design is well suited to examine treatments with small effect sizes in a general population with increased efficiency, particularly those treatments with a greater expected effect in a particular subpopulation of subjects.