Soft pneumatic actuators produce more energy output per unit mass than conventional rigid pneumatic actuators and are safer for applications involving physical contact with users or fragile objects. The design, modelling, fabrication, and optimization of origami-inspired soft pneumatic actuators (OSPA) are investigated in this thesis. A novel fabrication method employing heat shrinkable polymers conforming to reusable 3D printed molds is proposed. It is rapid, cost-effective, and more systematic than prior OSPA fabrication methods. A nonlinear finite-element analysis (FEA) model for an OSPA based on the accordion crease pattern is developed for predicting the actuator's folding behavior and blocked force. The model includes a nonlinear hyperelastic model of the heat shrink material’s behaviour (obtained empirically) and nonlinear frictional contacts. It is validated with experimental results and is shown to predict the blocked force with a 5.7% maximum error. Prototypes of two OSPA designs (accordion and Yoshimura patterns) are fabricated. Isometric, isobaric, isotonic, and cyclic fatigue tests are performed on the accordion pattern OSPA. The tests demonstrate that it can lift more than 124 times its own weight, and had no decrease in performance after 150,000 contraction/extension cycles with a payload of 2 kg. This durability is superior to existing OSPA. Lastly, a FEA model-based design optimization approach is proposed. A multi-objective genetic algorithm (MOGA) is used to find the origami design parameters that maximize the accordion pattern OSPA's work output. The optimized design is validated experimentally. Although this research focuses on the accordion pattern OSPA, the proposed fabrication, modelling and optimization approaches can be easily adapted to other OSPA designs. In addition to linear force and motion, these actuators can be combined to produce different motions, e.g., a pair of actuators can be connected by a cable to a pulley in an agonist-antagonist arrangement to produce a bidirectional rotary actuator.