Continued progress in many areas of science depends on access to advanced modern instruments and the data they provide. Costly instruments have been shared in a number of disciplines for many years, and common patterns of shared usage have developed independently. The scientific and financial aspects of large instrument usage are discussed from the points of view of the instrument centers, the users, and the funding agencies. The instrument problem is not one problem but many, and coordinated solutions must be implemented with well-defined goals based on knowledge of the needs of the users and developers of instruments.