This thesis presents a new methodology for frequency and duration (FeD) assessment in composite generation and transmission reliability evaluation of large scale power systems. The FeD indices are obtained from a nonsequential Monte Carlo simulation scheme. Two tecniques are developed to estimate the feD indices. In the first, the Lagrange multipliers associated to the adequacy assessment of each state are used to identify the boundary wall between failure and success states. The second approach uses the concept of conditional probability and system coherence to caracterize the contribuition of each component to the frequency indices. Also, the first technique is used to calculate upper bounds of these indices. The methodology is illustrated in case studies with three test systems, with 24, 120 and 500 buses, derived from real systems. The studies indicated that both tecniques are very attractive in estimating the FeD índices. The first approach was about 30% faster than the second one. On the other hand, the second approach is more flexible and easy to implement. The proposed approach allows the pratical utilization of frequency and duration indices in integrates generation/transmission planning studies.