Substituted spiro[isobenzofuran-1,4-piperidin]-3-ones and 3H-spiroisobenzofuran-1,4-piperidines capable of modulating NPY5 receptor activity are provided. Such compounds may be used to modulate ligand binding to NPY5 receptors in vivo or in vitro, and are particularly useful in the treatment of a variety of disorders (e.g., eating disorders such as obesity or bulimia, psychiatric disorders, diabetes and cardiovascular disorders such as hypertension) in humans, domesticated companion animals and livestock animals. Pharmaceutical compositions and methods for treating such disorders are provided, as are methods for using such compounds for detecting NPY5 receptors.