An assay is a scientific test used to determine the presence, amount, or activity of a substance in a sample. This process is commonly used in fields like chemistry, biology, and medicine to analyze materials such as blood, soil, or pharmaceuticals. Assays can help identify the concentration of specific compounds, assess the quality of products, or evaluate biological responses. There are various types of assays, including enzyme-linked immunosorbent assays (ELISA), polymerase chain reaction (PCR), and mass spectrometry. Each method has its own advantages and is chosen based on the specific requirements of the analysis. Assays play a crucial role in research, diagnostics, and quality control.