This review provides an overview of the drug discovery process used to identify, develop and characterize the first positive allosteric modulators of the metabotropic glutamate receptor (mGluR) subtype 5 (mGluR5). Discovery and optimization of three series of positive allosteric modulators are described, each using different approaches. The symmetric benzaldazine series was discovered and optimized from samples already existing in our sample collection without an active synthetic program to further elucidate SAR. This series yielded a family of highly selective pharmacological tools that produced positive, negative and neutral allosteric modulation of mGluR5 activity. The original compound in the benzamide series was discovered from screening and this series was optimized using an iterative library synthesis approach to explore SAR in each of three regions of the molecule. This series produced more potent positive allosteric mGluR5 modulators than the benzaldazine series which could be evaluated for their effect on mGluR5 in brain slice electrophysiological studies. The pyrazole series used a fragment library approach based on small structural motives from the benzamide series to discover lead compounds and establish SAR. This series produced still more potent positive allosteric mGluR5 modulators with improved pharmacokinetic and physical properties. These modulators showed efficacy in animal behavioral models in which other antipsychotic drugs were active. Evaluation of assay data in mathematical models of allosterism to constrain possible mechanisms of action is briefly discussed. Other reviews of this emerging field with different emphases have been published recently [1-3].