Some important design features which improve reactor core performance are currently being evaluated. The design predictions of reactivity effects associated with the use of Zircaloy clad fuel, burnable poisons, and part length control rods show good agreement with measurements made in recently commissioned reactors. Power distribution measurements are also compared with calculated values. A discussion of power distribution control and its relation to core performance is given. This discussion highlights the importance of whole-core calculations with analytical models whose accuracy has been experimentally verified.