We present a model of the interstellar medium in spiral galaxies. The ISM is simulated by a system of particles, representing gas clouds. It is a probabilistic N-body system in which “cloud” particles orbit ballistically, undergo dissipative collisions with other clouds, and experience velocity-boosting interactions with expanding supernova remnants (SNRs). Associations of protostars may form in clouds following collisions or SNR interactions (thus allowing sequential star formation); these associations take finite times before becoming active and undergoing their own SN events (Roberts and Hausman, 1983; Hausman and Roberts, 1983).