Analytic queueing models of programs with internal concurrency are considered. The program behavior model allows a process to spawn two or more concurrent tasks at some point during its execution. Except for queueing effects, the tasks execute independently of one another, and at the end of their execution, either wait for all of their siblings to finish execution or merge with the parent if all have finished execution. Two approximate solution methods for the performance prediction of such systems are developed, and results of the approximations are compared to those of simulations. The approximations are both computationally efficient and highly accurate. The gain in performance due to multitasking and multiprocessing is studied with a series of examples. Copyright © 1983 by The Institute of Electrical and Electronics Engineers, Inc.