The design of two passive back lift assist exoskeletons are presented in this thesis. The first exoskeleton uses carbon fiber as an energy storage medium while the second system utilizes a series system comprised of a gas spring and metal coil spring. The first exoskeleton was successfully tested long term in a warehouse environment and in laboratory experiments. From these tests and feedback from wearers several drawbacks to the design were discovered. Version two of the exoskeleton successfully addresses these concerns. / Master of Science / This document presents the designs of two lightweight passive exoskeletons. Exoskeletons are wearable devices that assist users in performing tasks that may be difficult or impossible without extra assistance. The exoskeletons developed and discussed in this thesis assist the wearer when bending or performing lifting tasks. As a user bends over or squats, energy is stored in the exoskeleton and is released when the user returns to standing. The first exoskeleton utilizes carbon fiber leaf springs to store the energy. It was successfully tested in a real-world warehouse setting and under laboratory conditions. Testing results and feedback from users led to modifications and new features which are included in the second version. The second exoskeleton uses a gas spring and coil spring in series for energy storage. It also incorporates a novel walking differential which allows users to seamlessly transition from walking to lifting.