During the last two decades we have entered a "golden era" of cosmology. Using satellites and ground based telescopes we have gathered high quality data from the very early Universe, essentially from light emitted right after the Big Bang explosion, as well as from the late Universe, through the light emitted from stars and galaxies. However, a big part of our Universe's history and volume remains unexplored. A way to attack this challenge is by observing the light emitted from the neutral hydrogen (HI) that filled the Universe for a long time after the Big Bang and before the first galaxies were formed. After that time HI resides within galaxies, so we can also use it as a novel way to study the late Universe. This is my main area of research; it is exciting because it opens a new observational window into the Universe and can push the boundaries of our understanding of astrophysics and cosmology. In the next few years, HI surveys of exquisite sensitivity will be performed using radio telescopes, and part of the proposed research is working on new techniques in order to maximise their science output. I have pioneered a new observational method that does not require the -difficult and expensive- detection of individual galaxies but maps the entire HI flux coming from many galaxies together in large 3D pixels (across the sky and along time). I aim to use this technique to provide a 3D map of the Universe using HI intensity mapping data from the MeerKAT and SKA arrays. MeerKAT is a radio telescope located in Karoo, South Africa, and it is a pathfinder for the Square Kilometre Array (SKA), which is going to be the largest radio telescope in the world. My main goal is to build a complete pipeline for the cosmological analysis of the HI intensity mapping signal from instruments like MeerKAT and the SKA. This pipeline will also account for the possibility of powerful synergies between HI and traditional optical galaxy surveys, by including cross-correlations data analysis tools. This is useful in order to obtain measurements that are free of systematic contaminations that often plague individual surveys (but drop out when combining them), and therefore more robust. I aim to perform the first ever measurements of HI and cosmological parameters in the radio wavelength using the intensity mapping technique, exploit multi-wavelength synergies, and revolutionarise our understanding of galaxy evolution and dark energy. I am also working on two of the largest and best optical galaxy surveys of the next decade, the Euclid satellite mission and the ground-based Large Synoptic Survey telescope, whose main goals are to measure dark energy and understand the initial conditions of the Universe. In Euclid, I am working on various projects including building the software tools that are going to be used for the analysis of the data as soon as they become available. I am also working on the very challenging task of modelling the way galaxies cluster on small scales, in order to extract useful information for cosmology. I am also using tailored simulations to assess how well Euclid will measure the largest cosmological scales in order to characterise the initial conditions of the Universe. In both Euclid and LSST, I am working on synergies with radio experiments. My goal is to find innovative ways to optimally combine optical and radio surveys, in order to maximise their joint scientific output. Another exciting part of working with these surveys is the huge amount of data that are going to be available. For example, the Phase 1 of the SKA is expected to generate 300 petabytes of data products every year. My research includes developing modernised and innovative data processing and analysis pipelines, which is required for the success of these amazing surveys.