How do galaxies form? How do they evolve? These are the fundamental questions driving the current deep, high-z galaxy surveys. A second approach, used here, reconstructs the star formation histories of galaxies from their resolved stellar populations. The main functions that are commonly used to define the star formation history of a complex stellar system are four: the star formation rate, the chemical enrichment law, the initial mass function and the binary function. The two that one expects to present a larger variation from system to system, and thus those defining its evolutionary history are the star formation rate and the chemical enrichment law. The star formation rate is derived in detail from deep color-magnitude diagrams. The chemical enrichment law was traditionally constrained from color distribution of RGB stars. However, this method to derive metallicities from photometry is a very crude one because in the RGB there is a degeneracy between age and metallicity. The main objective of this work is to break this degeneracy by obtaining stellar metallicities from another source and then derive the age from the positions of stars in the color-magnitude diagram. The best way to obtain stellar metallicities is high-resolution spectroscopy. However, to evaluate a suitable number of stars, a lot of telescope time is necessary. The alternative is low-resolution spectroscopy, which allows us to observe a significant number of stars in a reasonable time with the modern multi-object spectrographers. In galaxies, the only objects that can be observed spectroscopically are the most brilliant stars, which usually are RGB stars. The appropriate spectroscopic index to obtain the metallicity of these stars is the infrared Calcium II Triplet, (∼8500A) which is the main feature of the infrared spectra of red giant stars. The relationship between the equivalent width of the Ca II Triplet lines and metallicity has been studied in metal-poor and coeval stellar systems. However, galaxies have in general a wide ranges of ages and metallicities. The first step in this work was to study the behaviour of the Ca II Triplet lines with age and metallicity. For this purpose we have observed a sample of stars in open and globular clusters which cover the widest age and metallicity ranges, 0.25≤(Age/Gyr)≤13 and -2.2≤[Fe/H]≤+0.47, in which the behaviour of the Ca II Triplet lines has ever been studied. The next step in this work has been to measure stellar metallicities with this index in the Magellanic Clouds with the purpose of studying their chemical evolution. Thanks to their proximity, these galaxies are an ideal laboratory to test the power of this method. They are easily observed from the ground and their stellar populations have a wide range of ages and metallicities. The main result of this work is that both galaxies have a stellar population gradient, in the sense that the metal-rich stars are also younger, and they are concentrated in the central regions of both galaxies. On average, the Large Magellanic Cloud is more metal rich than the Small one.