Posttranslational modification by ubiquitin and SUMO is recognized as an effective means of controlling the stability, localization or activity of intracellular proteins, thereby contributing to the regulation of many biological processes. Over the past few years, it has become apparent that the two modification systems often communicate and jointly affect the properties of common substrate proteins, in some cases by being targeted to the same site. However, although SUMO and ubiquitin might have very different effects on a given target, their actions can rarely be explained by simple competition. This article gives an overview of target proteins that can serve as substrates for both SUMO and ubiquitin to highlight the diversity of regulatory strategies that result from the crosstalk between the two modification systems.