A one-dimensional space-dependent dynamic analysis of boiling water reactors, for direct, indirect or dual cycle systems with forced or natural circulation is presented. The analytical model consists of space-dependent neutron kinetics equations for the reactor core, and flow-conservation equations for the reactor coolant system developed in terms of length along the flow path and time. The resulting set of non-linear partial differential equations is expressed spatially in finite-difference form and integrated numerically in time to obtain the space- and time-dependent system variables. The effect of system-pressure variation is neglected. The mathematical model and numerical procedures employed in this study are verified against available test data from the Levy and Beckjord experimental boiling loop. Analytical predictions of the threshold of instability and the frequency of oscillations are shown to be in agreement with the test data. Studies of the uncontrolled and controlled behavior of a 110-MWe direct cycle boiling water nuclear power station confirm that, in contrast with natural-circulation loops, forced-circulation boiling systems have a high degree of hydrodynamic stability. However, an inappropriate selection of control-system parameters may induce nuclear power instability in both natural- and forced-circulation plants. The theoretical approach presented may be successfully employed as a powerful tool for the determination of the system stability, as well as for evaluation of the degree of effectiveness and relative merits of various system power-control techniques.