Enzyme-linked immunosorbent assay (ELISA) is one of the most specific and straightforward assays for detecting biomolecules in research and clinics. With advances in analytical methods, ELISA assay has been constantly optimized to improve its sensitivity, and different types of ELISA are now available to detect various biomarkers. This chapter provides an overall summary of the basic principle of ELISA, discusses different components of ELISA assay, and clearly outline protocols for different types of ELISA assays, including direct, indirect, sandwich, competitive, and nanoparticle-based ELISA.