My thesis project consisted in the data reduction and analysis of a set of Ks-band data. The near-infrared (NIR) observations were performed in the context of XMM-LSS, an X-ray survey with the main goal of studying the Large Scale Structures (LSS) of the Universe using ESA's satellite XMM--Newton. Multi-wavelength observations, coming from Legacy surveys associated to XMM-LSS, allowed not only to perform a detailed study of the properties of the selected population but also understand in-depth the selection process followed.