In many transduction processes, an increasing number of enzymes and other molecules become engaged in the events that proceed from the initial stimulus. In such cases the chain of steps is referred to as a "signalling cascade" or a "second messenger pathway" and often results in a small stimulus eliciting a large response. Hormones and other signalling molecules may exit the sending cell by exocytosis or other means of membrane transport. The sending cell is typically of a specialised type. Its recipients may be of one type or several, as in the case of insulin, which triggers diverse and systemic effects. This book sheds new light in this exciting field of cell transportation research.