The permeation process consists of the steps: adsorption, solution, diffusion, and desorption. These are discussed for glasses, polymers, and metals. There are two generalizations: (1) All gases go through all polymers. (2) No rare gas permeates any metal. Criteria are given for measurements of the permeation process.In glasses the permeation rate depends on atomic size and glass composition. Some special processes which may simulate permeation in vacuum devices are discussed: oxide dissociation, electrolysis in ceramic materials, nuclear reactions, and ionic impact.