The basics of in situ hybridization have been widely applied to a diverse range of situations where we need to localize the distribution of nucleic acids. Advances in other molecular techniques such as the advent of gene microarrays has not diminished the significance of in situ hybridization, but rather highlight the importance of being able to identify the topology of gene expression. In situ hybridization offers a degree of precision that is unavailable with other molecular techniques. This chapter outlines techniques used to examine the spatial distribution of gene expression in the kidney using complementary RNA (cRNA) probes with both radioactive and non-radioactive labels.