RNArchitecture is a database that provides a comprehensive description of relationships between known families of structured ncRNAs, with focus on sequence and structure similarities. RNArchitecture also provides literature information and links to other databases, including RFAM and PDB. Sequence relationships are defined according to multiple sequence alignments, and structure relationships are defined according to structure superpositions.The central level of Family describes evolutionarily related RNAs with conserved structure and detectable sequence similarity. Families that exhibit structural variation are further subdivided into subfamilies. Families with divergent sequences that share similar structures and functions and are almost certainly evolutionarily related are grouped into Superfamilies. Architecture describes geometrical relationships and groups together similar RNA structures that may be derived either by divergent or convergent evolution. The highest level Class, organizes Families into very broad structural categories, such as simple or complex structured RNAs or lncRNA domains with unknown structure.