DNA may be viewed as the blueprint for the body, with mRNA/protein the component parts made from this blueprint. However, it is only when proteins are allowed to interact with each other and their surroundings that true biological complexity is achieved. Thus, while it is informative to study transcriptional control and mRNA transcript levels, it is equally important to assess the impact of the encoded proteins on the total cellular environment. For example, expression of a ligand-activated receptor such as the epidermal growth factor (EGF) receptor is of no biological consequence if no EGF is present in the system. It is thus important to be able to study protein interactions and modification, enabling a comprehensive understanding of the biological mechanisms that underlie any particular phenotype. This article will outline the basic technologies to both visualize protein localization and interaction between co-localized proteins. In addition, the manipulation of protein levels, both in vitro and in vivo, will be described, as this provides an important tool for the further examination of protein functionality within biological systems