The paper presents the concept of habitat. It is a bounded chunk of space/time that isdesigned to accommodate a delimited set of activities. It accommodates the activities by in-cludingphysical artefacts that can be used in the activities and signs that offer activity-relevantinformation. The habitat concept was originally proposed as a help to understandmobile context-sensitive technology, but turns out to have much broader applications. Thepresent version of the approach uses activity theory and semiotics as the basic theoretical un-derpinnings.The paper offers a notation for specifying habitats, and gives examples fromcontext-sensitive mobile systems, 3D-simulations and games, and a number of everyday ac-tivitiesfrom the work domain.