This paper presents a solution methodology for the short-term transmission expansion planning problem based on the use of Genetic Algorithms (GAs). In this methodology, the overall problem is subdivided in an investment sub-problem and in an operation sub-problem, which are solved in a successive and iterative way. In the first sub-problem, a GA is used to choose the values of the expansion variables (location and quantity of new lines, capacitor banks, etc.). In the second sub-problem, simulations are performed to decide whether the system variables in the expanded electrical system satisfy the operational constraints. The methodology was tested on Garver's system and on a reduced configuration of the electrical Brazilian system.