In development since 1996, AlgaeBase (http://www.algaebase.org) is an on-line database providing free access to authoritative taxonomic, distributional and nomenclatural information of more than 135,000 names of species and infraspecific taxa of algae set in the context of a taxonomic hierarchy. The project was initially funded by the Higher Education Authority, Department of Education and Science (Ireland) and the European Union (the SeaweedAfrica Project), and more recently by an industry sponsor in Ireland (Ocean Harvest Technology) and various phycological societies and organisations. The database currently includes more than 50,000 bibliographic references and incorporates the entire contents of the main phycological journals in addition to taxonomic, ecological, physiological and biochemical references in current and classical works. Nearly 10,000 PDFs are included, many of them of 19th-century taxonomic works that are rare and difficult to obtain. The data are searchable at all taxonomic levels from kingdom to species (and infraspecific names), and AlgaeBase strives to provide citations of the original publications of all taxa. For any of the 145,000 taxa (names of genera and above included), all subordinate taxa at the next lowest rank are indicated along with the number of species for each. Within each genus the species and infraspecies taxa are listed along with the current taxonomic status of each name. Nearly 17,000 images are provided for downloading and use in teaching or research, with copyright and other rights being retained by the original contributors or by AlgaeBase. This database is being used by 2,000-3,000 individual visitors each day with nearly 100,000 requests a day and receives over 7 million “hits” each year, increasing at about 20% per annum. A brief description of other main on-line algal resources such as Index Nominum Algarum, the Catalogue of Diatoms Names, CyanoDB, and AlgaTerra is provided.