The long-term goal of this research is a prosthetic socket for trans-tibial prosthesis users that continually adjusts size so as to both maintain a proper fit and stabilize limb volume. The socket relieves users of the burden of continually sensing if their socket fit has deteriorated, deciding what adjustment to make, and effecting a socket size change. Users may instead focus on other aspects of their life. The auto-adjusting socket should enhance independence, improve limb health, and enrich patient quality of life. The specific aims are to modify an auto-adjusting socket for Walking previously developed by our group to add a remote key fob interface and two new auto-adjustment algorithms. The compact key fob eliminates a barrier present in the prior design. A Low Activity auto-adjustment algorithm manages socket size to maintain fit during short bouts of walking mixed with standing, as would occur for example when moving about in a building. A Sitting algorithm facilitates limb fluid volume recovery during sitting. The algorithms are optimized through lab- based participant testing. Performance of the complete auto-adjusting socket system is tested in user free-living environments in three modes: auto-adjusting; manual-adjusting; and locked (sock changes only). The aims are accomplished by alternating between clinical studies and engineering design. In the first aim, prototype key fobs are fabricated and evaluated by participants with limb amputation and prosthetists in a qualitative study design. From the results a single design is specified for development. In the second aim, new auto-adjustment algorithms are created and added to the existing on-board control system to maintain fit during combined short bouts of walking and standing (Low Activity) and during sitting (Sitting). Fit is monitored using measurements from custom limb-socket distance sensors embedded in the socket. In a crossover randomized trial, socket comfort, residual limb health, variability in limb fluid volume, and control system error are assessed while participants wear the socket during a structured protocol with all three auto-adjustment functions enabled (Walking, Low Activity, Sitting) compared with only the Walking function enabled. Based on study results, the socket control system is updated accordingly. In the third aim, a crossover randomized trial is conducted in at- home testing where participants wear the socket in auto, manual, and locked modes. In the manual mode, participants adjust the socket using the key fob. Participant self-report scores, residual limb health, prosthesis use, activities, socket fit, number of manual socket adjustments and participant preference are compared. The health relatedness of this application is a reduction of the burden of socket fit monitoring and fit management for patients with limb loss. Rather than having to continually manage their disability, people with limb amputation may live more independent lives. The auto-adjusti...