The rapid growth in immunochemical methods is attributed in part to the availability of antibodies for a variety of compounds of environmental significance. Immunochemical methods, which are usually simple to perform, are powerful analytical techniques that can answer many questions concerning plant pathology and environmental contamination. This chapter gives a general introduction of the principles of immunoassay, and outlines the practical realization of such assays. It addresses applications of immunochemical technology for the determination of toxins with special emphasis on mycotoxins and pesticides. Although a number of immunochemical methods have been used for the analysis of low-molecular weight compounds, two major methods, namely radioimmunoassays and enzyme immunoassay, have been developed for the analysis of mycotoxins and pesticides. The chapter discusses advantages and limitations of immunoassays, and includes potential applications and future research areas for immunochemical methods.