This textbook brings together and puts into perspective current knowledge of intra-cellular signalling systems within animals. The ordered growth and functioning of animal cells requires the transfer of information from one part of the cell to another. This information comes from a variety of sources, both within and outside the cell. Hormones, neurotransmitters, growth factors and extra-cellular metabolites interact with specific receptors on the plasma membrane or in the cytoplasm. Inside the cell, information is relayed by intra-cellular messengers including cyclic nucleotides, inositol polyphosphates, diacylglycerol and calcium, GTP-binding proteins, protein kinases and DNA-binding proteins. Finely balanced interactions between these different signalling pathways ensure that the required response occurs and abnormalities in any of the components of the system can lead to a variety of diseases, including cancer.