Ribosome is a universally conserved macromolecular machine responsible for protein synthesis. It is a ribonucleoprotein complex composed of two subunits and built of multiple proteins and ribosomal RNAs. The small subunit decodes the information stored within the messenger RNA (mRNA) whereas the large subunit catalyses the peptide bond formation in a growing polypeptide. Despite being universally conserved, ribosome composition differs significantly between prokaryotes and eukaryotes. Interestingly, eukaryotes possess more than one type of ribosomes. Beside the cytoplasmic ones, they also contain ribosomes specific to mitochondria (and chloroplasts, in case of plants). Mitochondria are double-membrane bounded cellular organelles that originated from α-proteobacteria in an endosymbiotic event and play a crucial role in production of cellular chemical energy and regulation of cellular metabolism. Mitochondrial ribosomes (mitoribosomes) are required for a proper function of mitochondria and defects in mitoribosomes are associated with a range of severe disorders in humans. Despite sharing a common ancestor, the structure of mitoribosomes differs significantly from their bacterial counterparts. In general, they contain a larger number of proteins, often specific to a given organism, and their ribosomal RNA is either reduced (humans and trypanosomes) or extended (fungi and plants). Nevertheless, the architecture of the ribosomes and the key functional regions in both subunits remain virtually unchanged and are similar to those observed in the bacterial ribosome. Assembly of ribosomes is a crucial and complex process that involves multiple steps and components. Over the years it has been studied both in bacteria and eukaryotes. However, our understanding on the mitoribosome assembly is still lacking. In my thesis I investigated the process of mitoribosome assembly by determining multiple structures of mitoribosomal large subunit assembly intermediates from Homo sapiens as well as human parasite Trypanosoma brucei. The structures reveal numerous assembly factors and explain their mechanism of action. In conclusion, results presented in this thesis provide structural insights into the complex and biomedically relevant process of mitoribosome assembly.