This book presents the research works and investigations related to the contingency due to line outage. Firstly, the detail assessment of the impact of line outage has been studied through PV curve method. Later on, investigation has been carried out to find the most critical line in normal loading condition. In view of this, ranking of the line outage contingencies are done on the basis of proposed overall severity index (OSI) comprising of two index i.e. active power deviation-based index and voltage deviation based index. Furthermore, the critical contingency is identified though well-known L-index. Moreover, the impact of critical contingency is analysed through new indices namely voltage deviation index (VDI) and loss deviation index (LDI). The effectiveness and viability of the proposed schemes are checked on different IEEE benchmark systems in MATLAB environment.