The primary purpose of lower extremity prostheses is to restore the locomotive functions of amputees’ lost sections and joints. The objective of this dissertation is to develop energetically active transtibial (TT, also known as below-knee) prostheses with powered ankle joints to generate sufficient power and torque with a compact form factor. Firstly, pneumatic sleeve muscle actuators have been designed and tested to investigate the feasibility of application of such type of actuator in TT prosthesis. Experimental results obtained on the prototypes validated and proved that sleeve muscle is a good fit for robotic systems with asymmetric force/torque requirements. Although the pneumatic sleeve muscle can provide sound force/torque capacities, it is challenging to build a transtibial prosthesis with this type of actuator that can match the size and weight of the human ankle. Hence, the subsequent efforts are directed towards utilizing a more compact actuation unit – pneumatic cylinder, which is well known for its capability in generating large force/power output with light weight and compact volumetric profile. Thus, a transtibial prosthesis has been designed using pneumatic cylinder. A finite-state impedance controller (FSIC) has been developed as a walking controller, and the parameters are tuned in walking experiments. The results from the experiments proved that the prosthesis is able to provide an improved gait compared with the traditional passive prosthesis. Additionally, a model that characterizes the stiffness and equilibrium point as functions of the chamber air masses in the pneumatic cylinder has been developed and a predictive pressure control algorithm was used to improve pressure control performance while minimizing valve actions. This enables the pneumatic actuator to be used as a variable series elastic actuator (VSEA). Experimental results showed that the proposed approach is able to provide the desired elastic characteristics of an artificial spring in stiffness control and demonstrated the advantages of this new approach for potential prosthetic applications. Lastly, the dissertation presents VSEA-powered TT prosthesis with direct implementation of the finite-state impedance control (FSIC). The human subject walking experiments were also conducted, and the results demonstrated the effectiveness of the direct FSIC prosthesis control approach.