Primary cilia are hair-like organelles expressed by almost every cell in the body. Although they were first recognized at the end of the 19th century, their functions remained obscure until the last decade. It is increasingly recognized that disorders of cilia structure or function underlie a number of rare human genetic diseases that affect the kidney and other organs, the so-called ‘ciliopathies'. Study of these cilia proteins is giving us new insights into how primary cilia function in normal human physiology and development and is shedding light on the pathogenesis of more common diseases, such as obesity, hypertension and organ fibrosis.