This document contains the study of "Microwave directional couplers". Initiating with a waveguiding system, we are looking for solutions of Maxwell's equations, then we focus on the TEM Transmission Lines and we introduce the microstrip lines that are used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate. In this chapter one, we also solve the equations, and discuss their solutions, describing coupled lines. In the second chapter, coupled microstrip lines are discussed. The configuration for these lines is shown in the document. We see the most important parameters that characterise a coupled microstrip line. Some of the important techniques for analyzing coupled lines are outlined. Coupled mode formulation and the even- and odd-mode method are described. In the last chapter we realize series of simulations of coupled microstrip lines with the CST STUDIO SUITE , program that allows us to make a more closely study. We can see how at the introduction of a transmission line more close to the other it is produced the coupling effect, transmitting the power with a proportion related with the fixed parameters that have been chosen. In the final of the paper we present the conclusions of all the simulations that have been realized and we expose the most representative ones in the annexes.