Diffusion – defined as mass, amount, or particle transport process – is the most essential transport property besides viscosity and thermal conductivity. Diffusion is widely used in nature, technical and scientific applications, ranging from diffusion in all kinds of gases, liquids, solids, electrolytes and their mixtures. Due to the large amount of data, the volume LB IV/15 “Diffusion in Gases, Liquids, and Electrolytes” is divided into three subvolumes. Part A: Gases in Gases, Liquids and their Mixtures. Part B: Liquids in Liquids and Liquid Mixtures. Part C: Ions and Electrolytes in Liquids, Electrolytes and Molten Salts. The presented subvolume A includes the main chapters Measurement Methods, Diffusion in Pure Gases, Diffusion in Gaseous Mixtures, Diffusion of Gas/Vapor in Gases or Liquids and Diffusion of Gases in Solution. Due to the very different and widespread measurement methods, a precise description of these methods is given.