Although first described as early as 1898 and long considered a vestigial 
organelle of little functional importance, the primary cilium has become one of 
the hottest research topics in modern cell biology and physiology. Primary cilia 
are nonmotile sensory organelles present in a single copy on the surface of most 
growth-arrested or differentiated mammalian cells, and defects in their assembly 
or function are tightly coupled to many developmental defects, diseases and 
disorders. In normal tissues, the primary cilium coordinates a series of signal 
transduction pathways, including Hedgehog, Wnt, PDGFRalpha and integrin 
signaling. In the kidney, the primary cilium may function as a mechano-, chemo- 
and osmosensing unit that probes the extracellular environment and transmits 
signals to the cell via, e.g., polycystins, which depend on ciliary localization 
for appropriate function. Indeed, hypomorphic mutations in the mouse ift88 
(previously called Tg737) gene, which encodes a ciliogenic intraflagellar 
transport protein, result in malformation of primary cilia, and in the 
collecting ducts of kidney tubules this is accompanied by development of 
autosomal recessive polycystic kidney disease (PKD). While PKD was one of the 
first diseases to be linked to dysfunctional primary cilia, defects in this 
organelle have subsequently been associated with many other phenotypes, 
including cancer, obesity, diabetes as well as a number of developmental 
defects. Collectively, these disorders of the cilium are now referred to as the 
ciliopathies. In this review, we provide a brief overview of the structure and 
function of primary cilia and some of their roles in coordinating signal 
transduction pathways in mammalian development, health and disease.