This dissertation presents an application of Probabilistic Load Flow (PFL) techniques to the expansion planning of power systems. A brief review of the PFL formulation and solution is included to identify differences between this technique and other available tools such as composite generation and transmission reliability evaluation algorithms. The potencial of the PLF technique is demonstrated by a case study using the Brazilian North/Northeastern system. The network expansion planning of an area of this system is studied using a conventional load flow program and the results compared with those obtained from a PLF program that models generation capacity unavailabilities and peak load uncertainties. The significant diffrences demonstrate the benefits of the PLF technique. Special emphasis is given to short and long term modeling using analyses of real system load data.