Abstarct: Current power systems are in a very tense situation due to factors such as increasing the penetration of renewable sources and the gradual growth of voltage-dependent loads. Therefore, voltage stability analysis and providing voltage stability indicators in order to determine the maximum load-ability is one of the important issues in power system stability. In addition, the interaction between transmission and distribution system operators are generally neglected due to the simplification in the load-ability analysis. In this thesis, by presenting a modified voltage stability indicator (MVSI) and load pattern-baxsed voltage stability index (LPVSI) in real-time analysis, voltage stability analysis in distribution systems has been performed. Also, in order to calculate maximum load-ability, a static load model baxsed on composite ZIP model and considering the two-tier load model is presented. In addition, increasing the penetration level of distributed generation units becomes the new challenge of traditional power distribution networks, especially local voltage increases. Hence, by presenting the load-ability sensitivity coefficient in the optimization algorithm, the maximum penetration level of DGs has been calculated. The results on IEEE distribution systems indicate the accuracy of the results compared to other studies. Moreover, voltage stability analysis and maximum load-ability prediction in integrated transmission-distribution systems with limitations of reactive power generated by transmission system generators as well as the interaction of DG penetration performance on transformer distribution systems are investigated. The results on the 14-bus IEEE transmission system with three interconnected distribution systems show that the load-ability of the distribution systems is strongly dependent on the interaction between the integrated transmission-distribution systems.