The relevance of the ubiquitin proteasome system (UPS) to disease and fundamental cellular processes has generated a demand for methods to monitor the activity of this system in living cells and organisms. Here we describe the GFP(u) family of UPS reporters. These reporters are constitutively degraded, ubiquitin-dependent proteasome substrates that can be used to monitor UPS function in the living cell. The GFP(u) reporter family consists of three variants that can report on global, nuclear, and cytoplasmic UPS function. This article focuses on the properties and design of these reporters and highlights appropriate techniques and applications for their use.