The ability to measure mRNA encoding protein is attractive, since, provided certain criteria are met, the investigator can be confident of highly selective detection. In addition, changes in mRNA expression as well as their precise cellular localization, provides the scientist with important information that may not be evident by the detection of the translated protein. Hence, the combined approach of assessing mRNA and protein expression will often allow a more precise hypothesis to be formulated. In the present chapter, we describe methods we have optimized for the detection of mRNA following extraction from tissue or cells (Northern hybridization) and the detection of specific mRNA transcripts within their synthesising cells (in situ hybridization).