Encyclopedias, Almanacs, Transcripts, and Maps
- International Primate Protection League
- International Register of Potentially Toxic Chemicals (U. N. Environment Programme)
- International Society for Environmental Ethics
- International Trade in Toxic Waste
- International Voluntary Standards
- International Water Laws and Enforcement
- International Wildlife Coalition
- interplanetary matter
- Inti
- intravenous drip
- Introduced Species
- Inuit Religious Traditions
- Ioann of Kronstadt
- Iodine 131
- ionic compound
- Ipatieff, Vladimir Nikolaevitch (1967–1952)
- Ippen
- Iqbāl, Muḥammad
- Iranian Religions
- Irenaeus