Living organisms use complex pathways of signal perception and transduction to respond to stimuli in their environments. In plants, putative signal transduction components have been identified through mutant screens and comparative analysis of genome sequences of model eukaryotes. Several pieces in a large series of puzzles have now been identified and a current challenge is to determine how these pieces interconnect. Functional analysis of the encoded proteins has necessitated a change from genetic to biochemical approaches. In recent years, the application of techniques such as two-hybrid screening and epitope tagging has facilitated the study of protein-protein interactions and has increased our understanding of cellular signalling mechanisms. One focus of present research is the ubiquitin/proteasome-mediated degradation of proteins. Increasing evidence suggests this is a control common to many plant signalling pathways including development and responsiveness to hormones, light and sucrose. A central challenge in the study of plant disease resistance has been to identify protein complexes that contain host defence proteins and pathogenicity factors. In this review we summarize the latest developments in these areas where the existence of protein complexes has been demonstrated to be of fundamental importance in plant signalling.