This paper examines gravity as a controllable environmental parameter. In long-duration orbital habitats and interplanetary vessels, designers face difficult decisions as to whether to provide artificial gravity, and if so, how much. Habitats on the moon and minor planets pose other issues. Partial gravity offers greater freedom of movement on the vertical axis, but may hinder floor traction and horizontal mobility. The minimum gravity required to preserve health remains unknown. Supplementing significant but insufficient natural gravity with artificial gravity is problematic. However, the presence of at least some natural gravity may offer other alternatives that are not available in orbit.