Ribosome biogenesis is a complex assembly process generating the cell’s protein factories consisting of four large RNAs and more than 50 proteins. As ribosomes are of critical importance for each cell, alterations in ribosome biogenesis lead to human diseases. In order to better understand ribosome formation on a molecular level, our research focuses on the early steps of this process where only a small number of factors are involved. In particular, we study the modification of functional RNAs such as ribosomal and transfer RNA both in bacteria and eukaryotes. By using highly purified components, we re-create this process under defined conditions in the test tube with the aim of identifying critical, limiting steps. Our approach is multi-disciplinary including molecular biology, biochemistry, biophysics and genetics allowing us to gain detailed insight into the functional cooperation of proteins and RNA in the assembly of cellular machines.