All vertebrates have characteristic asymmetries along the left/right axis. The positioning of asymmetric visceral organs is highly conserved evolutionarily and disruptions in left/right patterning can lead to severe morphological defects, demonstrating the importance of regulation of left/right developmental asymmetries. Our understanding of vertebrate left/right pattern formation has been advanced by studying several mouse mutations which disrupt this process. These mutant mice have served as tools to help us to unravel the genetic pathways of left/right development. The identification and analysis of genes with asymmetric expression patterns has allowed us to begin to understand the mechanisms which regulate left/right development.