The current state of space-heating commercial greenhouses with solar energy is reviewed. Selected projects from numerous locations in the U.S. are reviewed briefly, projects which illustrate the various approaches to solar heating. Techniques include: external collectors, integral greenhouse/solar collectors, and passive solar heating systems. Short-term as well as long-term thermal storage is being investigated. Although no project yet has a system ready for commercial adoption, several possess promising features. To be adopted, the solar system must: (i) provide a relatively short payback, (ii) not interfere with the ability of the grower to produce a high quality crop without excessive delays, and (iii) be coordinated with all reasonable energy conservation measures.