Many basic events which concern management and manipulation of genes within a cell – for instance transcription, replication and recombination – rely on specific interactions between proteins and nucleic acids. Such interactions are also essential for many house-keeping functions, like packing and unpacking of DNA in chromatin and assembly of ribosomes. Moreover, the details of protein–nucleic acid interplay is essential for understanding the action of viruses. The list of functional mechanisms in biology that rely on protein–DNA and protein–RNA interactions can be made much longer, but these examples represent some of the topics which motivated structural biologists to study complexes between proteins and nucleic acids as a first step beyond structure determinations of individual biomolecules.