Concomitant developments in molecular biology and neuroanatomy have permitted the development of techniques to visualize the expression of messenger RNA (mRNA) of interest in its neuroanatomical context and in a relatively quantitative manner. In situ hybridization can be performed using either radioactive or nonradioactive techniques and quantitated using specialized image analysis programs. This chapter will describe a sensitive radioactive method of in situ hybridization, with references given to other variants in technique.