``Clusters of galaxies are the largest virialised systems in the Universe'' is one of the most common statements in the framework of galaxy clusters research. This occurs because such gravitationally bound structures become ideal laboratories to directly study their baryon content (intracluster medium and galaxies) and to indirectly measure their dark matter content. In the present thesis we focused our attention on the galaxy population of clusters in order to shed some light about the process or processes that act on galaxies producing morphological transformations, enhancement/truncation of the star formation activity, colour changes, etc. We investigated the physical properties of the galaxy population of two distant clusters with the aim of constraining the physical mechanisms governing the evolution of clusters and their galaxies.

For this purpose, we have used the innovative observational technique of the OSIRIS/GTC tunable filters (TF) to detect galaxies with emission--lines. In particular, we presented the mapping of two emission--lines: the Halpha line in the Cl0024+1654 cluster at z~0.4 and the [OII] line in the RXJ1257+4738 cluster at z~0.9. Since it is an unusual technique, we included in this thesis a complete description of the procedures we have performed from the raw data until achieving the final cluster catalogues. The catalogues finally derived demonstrate the powerful of the TF tomography technique to obtain a reliable cluster membership sample. Our results proved that we are able to determine with high accuracy the line position, which allows us to compute radial velocity values comparable to low resolution spectroscopy measurements, and to derive efficiently the line flux, which allows us to estimate the star formation activity.

Moreover, in the particular case of Cl0024, we have revealed the potential of the emission--line galaxies to estimate, using the caustic method of Diaferio et al. (1997), the cluster total mass. For this cluster, the TF data also allow us to deblend the Halpha and [NII] lines, enabling the study of the active galactic nuclei population.

The analysis of the RXJ1257 cluster, the main target of this thesis, was made in three steps: the infrared view, the [OII] emission--line mapping, and the morphological characterization. We have also taken advantage of the TF technique, but the full study of RXJ1257 was possible through the use of a complete multi--wavelength dataset covering the range from the optical to the FIR. This multi--wavelength dataset is comprised by g'r'i'z' broad--band imaging from OSIRIS/GTC, J--band imaging from LIRIS/WHT, the four bands imaging of IRAC/Spitzer, 24um--band imaging of MIPS/Spitzer, the 100 and 160um bands imaging of PACS/Herschel, and the three bands imaging of SPIRE/Herschel. First, we built a cluster membership catalogue with ~300 cluster galaxies by fitting the multi--wavelength dataset to spectral energy distribution templates. Then, we analysed the FIR-- and the [OII]--emitter populations. As a result, we observed a filamentary--like substructure distribution with the population of emitters preferentially located at medium densities, suggesting that the RXJ1257 cluster is still under the process of formation. We found that an average star--forming galaxy forms the same stellar mass per unit time in all local density regions, but that the relative number of star--forming galaxies is significantly higher at medium-- to even high--density environments. Therefore, for this cluster, we noticed that the quenching of the star formation activity in the densest regions has not happened yet. Since the FIR and [OII] cluster populations do not completely overlap, we investigated the three star--forming samples ([OII]--only, FIR--only, and [OII]+FIR emitters) and obtained that the most plausible explanation to the lack of [OII]--emission in the most massive and reddest FIR--emitters is the attenuation by dust.

Finally, this thesis includes a detailed morphological classification of the RXJ1257 cluster members. This work used the non--parametric methods implemented in the galSVM code (Huertas-Company et al. 2008) to prove the feasibility of the partial classification of galaxies in clusters at z~0.9 with data from ground--based telescopes, such as the GTC. The analysis of the morphologically classified cluster galaxies indicated that even in such a young and distant cluster the early--type galaxies are concentrated in the densest regions, while the late--type tend to be located at some distance from the cluster centre.