This paper describes the characteristics and functionalities of the web-based database NAPROC-13 (http://c13.usal.es). It contains Carbon NMR spectral data from more than 21.000 Natural Products and related derivates. A considerable number of structures included in the database have been revised and corrected from the original publications considering subsequent published revisions. It provides tools that facilitate the structural identification of natural compounds even before their purification. This database allows for flexible searches by chemical structure, substructure of structures as well as spectral features, chemical shifts and multiplicities. Searches for names, formulas, molecular weights, family, type and group of compound according to the IUPAC classification are also implemented. It supports a wide range of searches, from simple text matching to complex boolean queries. These capabilities are used together with visual interactive tools, which enable the structural elucidation of known and unknown compounds by comparison of their 13C NMR data.