Recent findings shed new light on the process of receptor tyrosine kinase (RTK) activation and signal definition. In extension to the established mechanism of ligand-induced homodimeric receptor complex formation, recent findings highlight heterodimeric receptor aggregation as a powerful means of signal diversification. Promiscuous receptor interactions involve different ligand binding kinetics and generate divergent receptor phosphorylation sites that could allow enhanced or modified signal generation. Besides activation by a specific ligand, a newly defined RTK function involves signal integration of a variety of stimuli, including calcium-dependent responses in neuronal cells, activation of G-protein-coupled receptors or cellular stress such as UV irradiation. On the basis of existing evidence for such crossactivation pathways, RTKs must be considered as representing critical foci and switch points for multiple environmental and internal stimuli.