Neutral hydrogen in interplanetary space

The theory and observations relevant to the problem of neutral hydrogen in interplanetary space are reviewed. Emphasis is placed on those theoretical problems whose treatment in the existing literature is not entirely satisfactory, but discussion of all significant observational and theoretical aspects of the interplanetary H problem is provided. Attention is also given to other neutral constituents (particularly He) that are relatively abundant in interplanetary space and in the local interstellar medium, which is the primary source of the interplanetary neutral gas. Some consequences of the passage of the solar system through an interstellar cloud are also briefly considered.