The following categories exist on the wiki, and may or may not be unused. Also see wanted categories.
- Agricultural subsidies (1 member)
- Agriculture (624 members)
- Agriculture manual (21 members)
- Agrivoltaic (1 member)
- Agrivoltaics (38 members)
- Agroforestry (9 members)
- Agua y Suelo para la Agricultura (ASA) (63 members)
- Aid (11 members)
- Aid and development workers (20 members)
- Air Quality (1 member)
- Air conditioning (16 members)
- Air pollution (36 members)
- Air purification (1 member)
- Air quality (20 members)
- Air travel (4 members)
- Akvo ATOC (10 members)
- Akvopedia (1 member)
- Alcohol fuels (20 members)
- Algae (5 members)
- Alternative building (122 members)
- Alternative building design (4 members)
- Alternative energy (1 member)
- Alternative propulsion (3 members)
- Amaranth (12 members)
- Ambiental (8 members)
- America files (33 members)
- America images (0 members)
- AmoSmile (61 members)
- Amsterdam (1 member)
- Anaerobic digestion (33 members)
- Analysis (8 members)
- Anatomy (7 members)
- Andhra Pradesh (2 members)
- Android App Browser Testing (1 member)
- Animal husbandry (13 members)
- Animal power (7 members)
- Animals (31 members)
- Apiculture (4 members)
- Applied Thermodynamics (1 member)
- Appropedia (36 members)
- Appropedia's Public Domain Search (15 members)
- Appropedia, Appropedia Foundation (1 member)
- Appropedia Action Groups (0 members)
- Appropedia Foundation (12 members)
- Appropedia Help (1 member)
- Appropedia WordPress site (1 member)
- Appropedia administration (18 members)
- Appropedia archive (205 members)
- Appropedia books (0 members)
- Appropedia bots (5 members)