The aim of this chapter is to provide a detailed and practical introduction to immunoassays and set the overall context for the other detailed chapters on specific key elements of immunoassays. The production of antibodies, various antibody structures and their fundamental role in immunoassays is outlined. Comprehensive guides to different immunoassay formats ranging from direct to competitive are provided, and subsequently, important signalling systems, including colourimetric and fluorescence-based approaches, are examined. Finally, electrical, mechanical and optical signal transduction mechanisms that are used in the next generation of immunoassays are described.