Modal cost analysis furnishes a promising methodology for developing dynamical models of space structures for use in control systems analysis. Economy and accuracy can be attained by only retaining vibration modes that contribute significantly to an appropriately defined “cost” function. Expressions for modal costs (especially simple for “lightly damped” structures) are derived for attitude control, vibration suppression, and shape control. These techniques are illustrated through application to a high-order finite element model of a large platform-type structure. © 1982 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.