Several lines of evidence show that induction of immediate early genes (IEGs) is a crucial step in regulating long-term plasticity-related changes at the cellular level. The term immediate early gene aptly characterizes that these genes are induced rapidly, within minutes, and transiently following neuronal activation. This is because their activation is not dependent on de novo protein synthesis as other, late response genes require. Such rapid induction of these genes offers a unique and powerful way to examine how a specific stimulus can lead to long-term cellular changes. By mapping IEG mRNA or the resulting protein expression, researchers have investigated patterns of brain activation in response to various stimuli and behavioral paradigms. In addition, IEG activity can be used to examine structural and functional aspects of brain hemisphere lateralization. This chapter will discuss several IEGs, but our specific method will focus on zenk. Zenk is a widely studied IEG in birds and has been used to identify specific brain regions critical for song learning, memory, homing behavior, and conspecific recognition. Here, we describe how standard immunohistochemical techniques and widely available imaging software can be used to visualize ZENK protein expression at the cellular and macro levels, such as whole sagittal or coronal brain sections. These visualization techniques provide the investigator with a powerful tool to examine functional asymmetries in the brain.