Technical Abstract: Due to the increasing number and quantity of organic pollutants, regulatory authorities require implementation of rapid, reliable, and cost-effective technologies for monitoring of water quality. Immunoassays provide a simple, powerful and inexpensive method for monitoring organic contaminants in bodies of water. However, the acceptance of immunoassays relies on the demonstration of quality and validity comparable to sophisticated instrument techniques. In this chapter, fundamentals and design of immunoassays for organic contaminants are outlined with emphasis on factors affecting assay performance, especially of ELISA tests. Specific examples are given to further understanding of the high selectivity and sensitivity of immunoassays for common organic pollutants (mainly pesticides and personal care chemicals) in water. A distinction is made between immunoassays that are quantitative and others used for qualitative screening. Cross-reactivity, the matrix effect, and data interpretation are illustrated by a number of examples from the literature. Quality assurance and guidelines for validation and use are given. Some newly developed immunosensor methods using surface plasmon resonance, quartz crystal microbalance, or electrochemical transduction are briefly reviewed.