An extensive study of boiling water reactor scram reactivity behavior is presented. It is based on a spacetime analysis using a two-dimensional (R,Z) dynamics code that includes a two-phase thermal-hydraulics model. Calculations were made of the sensitivity of scram to such physical quantities as initial control rod position and power distribution, scram speed, system pressure, and varying inlet flow rate and temperature. The end-of-cycle Haling operating condition with all rods initially withdrawn was found to give rise to the limiting scram reactivity function. Calculations were also made to find the effect on scram of commonly used modeling approximations. These included the effect of neglecting delayed neutrons (conservative), using a time invariant void distribution (nonconservative), and defining point kinetics parameters in terms of different weighting functions. The importance of defining these parameters consistent with their use in plant transient analyses was also demonstrated.