'Ciliopathies' are an emerging class of genetic multisystemic human disorders that are caused by a multitude of largely unrelated genes that affect ciliary structure/function. They are unified by shared clinical features, such as mental retardation, cystic kidney, retinal defects and polydactyly, and by the common localization of the protein products of these genes at or near the primary cilium of cells. With the realization that many previously disparate conditions are a part of this spectrum of disorders, there has been tremendous interest in the function of cilia in developmental signaling and homeostasis. Ciliopathies are mostly inherited as simple recessive traits, but phenotypic expressivity is under the control of numerous genetic modifiers, putting these conditions at the interface of simple and complex genetics. In this review, we discuss the ever-expanding ciliopathy field, which has three interrelated goals: developing a comprehensive understanding of genes mutated in the ciliopathies and required for ciliogenesis; understanding how the encoded proteins work together in complexes and networks to modulate activity and structure-function relationships; and uncovering signaling pathways and modifier relationships.