The list of human disordered associated with cilia dysfunction, the ciliopathies, continues to highlight the importance of understanding the many roles of the long overlooked primary cilium. Much of the insights into the clinical importance of the cilium have come from analyses in model organisms, especially the mouse. However, the early embryonic lethality and severe developmental defects associated with cilia disruption has hindered progress in exploring cilia functions in late development or in adult tissues. This hurdle is being surmounted through the use of conditional alleles of genes encoding ciliary proteins and Cre deletor lines with inducible Cre activity or with lines expressing Cre in a cell-type-specific manner. Results from these approaches are providing important insights into the diverse array of cellular and tissue activities regulated by the cilium. Here we provide a recent account of the Cre/lox strategy. The generation and use of well-designed conditional alleles, as well as careful manipulation of embryonic stem cells are discussed. We also provide specific examples to illustrate the use of Cre/lox approaches to evaluate ciliary function in several tissues. With the recent characterization of multiple cilia proteomes along with efforts of several consortia to generate conditional alleles of all genes in the mouse, further use of conditional mutation approaches promise to yield many advances and surprises as we explore the functions of this increasingly complex organelle. 2009 Elsevier Inc. All rights reserved.