Electrophoresis usually utilizes some form of solid support to prevent convectional distortion of the analyte bands and is found in the paper form and as polyacrylamide or agarose gels. Capillary electrophoresis (CE) offers high throughput, high resolution, automatic operation, and on-line detection with automatic data acquisition. This chapter discusses the fundamental theory, equations, and definitions necessary to comprehend the concepts involved in CE. It describes the separation mechanisms of electrophoresis. The main modes of CE are capillary zone electrophoresis, micellar electrokinetic capillary chromatography, capillary electrokinetic chromatography, capillary isoelectric focusing, capillary gel electrophoresis, and capillary isotachophoresis. These mechanisms help the researcher to understand the methodology, analyze the experimental data, identify the relevant separation regimes, and/or design the optimization strategies. Capillary electrochromatography is a technique that has high separation efficiency and selectivity. Absorbance detectors are the most commonly encountered detectors in CE instruments. They measure the absorbance of light energy by the analytes.