This thesis investigates the development of a dispatch/pricing model to examine the effect of maintaining voltage stability margins on spot prices, and also presents models for voltage stability constrained reactive power planning. We develop an AC OPF primal problem incorporating both multiple generator contingency constraints and multiple voltage stability margin contingency constraints. Active and reactive power trade off functions for the generators and the opportunity cost of foregoing active power generation to provide increased reactive power are considered. The objective is to minimise costs while maintaining the capability to deal with pre-defined contingency events. The dual of the primal problem is analysed to determine generation, reserve and demand price equations, which are examined to establish the effects on active and reactive power spot prices of generator contingency constraints and voltage stability margin contingency constraints. We also develop a voltage stability constrained and contingency constrained reactive power planning model. This uses a non-linear mixed integer programming algorithm to efficiently formulate and solve the VAR planning problem. Next, a statistical approximation procedure simplifying the voltage stability constrained planning model is presented. An equation of the locus of the PoCs is developed using statistical regression methods and used to simplify the full MINLP model. Finally, a preventive/corrective control model is developed to guard against voltage instability while taking into account the load-shed dynamics.