The protein tyrosine kinases (PTKs) are a large and structurally diverse family of enzymes. The conserved catalytic domain held in common by each member of this family is a self-contained 250-300 amino acid unit bearing sixteen highly conserved linear sequence elements, several of which have been shown to be important to the catalytic activity of this domain. The enzymic activity of the PTKs is clearly an evolutionarily successful theme, and at least 10 distinct morphotypes have been described. Many of these resemble cell surface receptors for growth factors, and for a small sub-set of these receptors a ligand has been discovered. The remainder are located intracellularly and presumably sense and respond to appropriate metabolic cues by exerting their physiologically powerful enzymic activity. A detailed examination of the structure/function relationships of the PTKs and their catalytic domains is particularly revealing in trying to establish the roles that these proteins play in signal transduction in eukaryotic cells.