he integration of renewable energies in electrical power systems is a fully established fact, as well as its forecast for expansion planning in the upcoming decades. Its future expansion is expected to cover all generation, replacing the conventional thermal generation that currently participates in power systems. Therefore, the question that arises is whether renewable energy will be able to ensure the supply of electrical energy as it is presently, guaranteeing high availability at an affordable cost for all consumers. This increment in renewable penetration has a great influence on the reliability of the systems, mainly due to the great uncertainty associated with renewable technologies.This study arises from the need to know how the reliability of power systems will be affected, in terms of adequacy and flexibility. Adequacy is related to the capacity that the system needs to have in order to supply a certain demand, while flexibility is related to how the composition of that generation must be with the purpose of adapting the changes that occurs in demand. The knowledge of power system reliability is carried out through reliability analyses. Through these analyses, it is possible to assess which level of reliability future planned power systems will have, both medium and long term. This work develops the necessary methodology to carry out reliability evaluations that later will be applied to the proposed cases of studies to characterize the effect on the reliability of different technologies. To obtain a correct approach for reliability studies, in which the uncertainty of renewable energies is considered, it is required to apply a probabilistic analysis for each resource. This approach is complemented by the sequential Monte-Carlo calculation tool that is one of the most suitable for solving stochastic problems with a large number of random variables.