This document discusses Attenuated Total Reflectance (ATR) spectroscopy. ATR spectroscopy allows for analysis of opaque, thick, or liquid samples without sample preparation. It works by producing an evanescent wave at the crystal surface that penetrates the sample. Different crystal materials can be used as the ATR element depending on the sample properties. Common crystals include diamond, germanium, zinc selenide, and silicon. ATR spectroscopy provides fast, reproducible analysis of various material types with minimal sample preparation.