Obligate means "by necessity" (antonym facultative) and is used mainly in biology in phrases such as:
- Obligate aerobe, an organism that cannot survive without oxygen
- Obligate anaerobe, an organism that cannot survive in the presence of oxygen
- Obligate air-breather, an organism dependent on oxygen respired directly from the atmosphere rather than extracted from solution in water, for example, the South American lungfish
- Obligate carnivore, a true carnivore — dependent for survival on a diet of animal flesh.
- Obligate intracellular parasite, a parasitic microorganism that cannot reproduce without entering a suitable host cell
- Obligate parasite, a parasite that cannot reproduce without exploiting a suitable host
- Obligate photoperiodic plant, a plant that requires sufficiently long or short nights before it initiates flowering, germination or similarly functions
- Obligate symbionts, organisms that can only live together in a symbiosis
- Obligate hibernation, a state of inactivity in which some organisms survive conditions of insufficiently available resources.