In one arrangement, a first device presents a display that is based on context data, derived from one or more of its sensors. This display is imaged by a camera in a second device. The second device uses context data from its own sensors to assess the information in the captured imagery, and makes a determination about the first device. In another arrangement, social network friend requests are automatically issued, or accepted, based on contextual similarity. In yet another arrangement, delivery of a message is triggered by a contextual circumstance other than (or in addition to) location. In still another arrangement, two or more devices automatically establish an ad hoc network (e.g., Bluetooth pairing) based on contextual parallels. In still another arrangement, historical context information is archived and used in transactions with other devices, e.g., in challenge-response authentication. A great number of other features and arrangements—many involving head-mounted displays—are also detailed.