The general principles of seismic soil-structure interaction by finite element methods are discussed, and the theory and manual for a specific computer code, FLUSH, are presented. This code is a further development of the complex response finite element computer program LUSH (Lysmer et al, 1974). The new program is considerably faster than LUSH, thus the name FLUSH = Fast LUSH, and it includes a large number of new features such as transmitting boundaries, beam elements, an approximate 3-D ability, deconvolution within the program, out-of-core equation solver, new input/output features, etc., all of which make the program more efficient and versatile. This study includes analyses of soil-structure interaction effects during earthquakes for nuclear power plant structures. (GRA)