This work combines biochemical studies, structural biology investigations and small molecule design within some of the protein classes that play major roles in signal transduction: the protein kinases, the growth factor family and the transcription factors. It describes how these proteins communicate with each other via different signalling pathways. Three-dimensional structures of these proteins are presented, and their interaction with small molecules and co-factors are described. The use of these proteins for pharmaceutical research and the design of small molecules and their selectivity towards such proteins are discussed.