Beginning with the basic concepts of a biological signal and receptor the definition of signal transduction is explained. We then explore various ways in which cells signal other cells: paracine, synaptic and endocrine. Extensive use of animated graphics brings the processes of signal processing to life, showing how the various types of signalling molecules enter cells and how their receptors operate, in particular channel-linked and G- protein receptors. Second messengers are explained. The final section deals with signal amplification, divergent and convergent pathways.