Rigorous assessment of uncertainties associated with capacitor installation in distribution systems and those associated with load demands is the aim of the present work. Interval mathematics provides a powerful tool for modeling such uncertainties. The present work developed in this book focuses on three approaches.The first concerns the assessment of uncertainties associated with capacitor allocation and sizing in distribution feeders using interval mathematics. The second approach involves the assessment of uncertainties in load flow using interval arithmetic. The third approach is based on modeling load uncertainties in reactive power compensation of distribution feeders using interval arithmetic.Implementation of the proposed technique is applied to different radial distribution feeders, one of them is a practical Egyptian feeder, and satisfactory results are obtained.