Several lines of evidence show that induction of immediate early genes (IEGs) is a crucial step in the regulation of long-term plasticity-related changes at the cellular level. The term immediate early gene aptly characterizes the fact that these genes are induced rapidly, within minutes, and transiently following neuronal activation. This is due to the fact that 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 activity or the resulting protein expression, researchers have been able to investigate patterns of brain activation in response to various stimuli and in different behavioral paradigms. In addition, IEG activity can be used to examine structural and functional aspects of brain hemisphere lateralization. Several IEGs will be discussed in this chapter but our specific methods will focus on Zenk, a widely studied IEG in birds that 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 level and at a macro level, such as whole sagittal or coronal brain sections. These visualization techniques provide the investigator with a powerful tool to examine asymmetries in the brain.