Post-translational modification by the attachment of ubiquitin seems to have a crucial role in regulating synaptic structure and function. By controlling the stability, activity and localization of target proteins, this versatile regulatory system can shape the pattern, activity and plasticity of synaptic connections. Here, we discuss the myriad ways in which ubiquitin functions to sculpt synapses during development, and to remodel synapses for the acquisition and storage of memory.