Capillary gas chromatography is gas chromatography performed in an open-tubular column of small internal diameter rather than in a column filled throughout with packing. The column wall carries either a thin immobilized liquid film, forming a wall-coated open-tubular column, or a porous adsorbent layer, forming a porous-layer open-tubular column ^[1]. An inert carrier gas transports vaporized solutes, and differential partitioning or adsorption produces separation according to volatility and stationary-phase selectivity. The absence of a packed bed reduces resistance to mass transfer and permits long columns with high plate numbers, while internal diameter, stationary-phase film thickness, carrier-gas velocity, and temperature program determine efficiency and retention ^[2]. The concept excludes packed-column gas chromatography, liquid chromatography in fused-silica capillaries, and gas-phase mobility methods without a stationary phase. Its necessary features are a gaseous mobile phase, an open capillary bore with a wall-bound stationary phase, controlled sample vaporization, and chromatographic migration along the column.