Macrocyclic lactones (MLs), exemplified by the prototype of the class, ivermectin (IVM), are mainstays of programs for the control of nematode and arthropod parasites and pests. Since their introduction 30 years ago, research has revealed that they act on a family of ligand-gated chloride channels gated by glutamate, which is largely restricted to animals in the phyla Nematoda and Arthropoda. Studies on IVM in model organisms have contributed greatly to our understanding of ML pharmacology, but our understanding of the basis for differences among species and among MLs in potency and spectrum remains far from complete.