The following categories exist on the wiki, and may or may not be unused. Also see wanted categories.
- Activism (24 members)
- Activist pages (3 members)
- Acute Appendicitis (6 members)
- Adaptación al cambio climático (1 member)
- Additive manufacturing (5 members)
- Adelaide (3 members)
- Adelaide files (1 member)
- Adelaide images (0 members)
- Aerosol pandemic (34 members)
- Africa (104 members)
- Africa files (29 members)
- Africa images (0 members)
- African American Environmentalist Association (16 members)
- 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 (35 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)