Compliant actuators are mechanical devices that extend the traditional architecture of robotic joints (motor – transmission – load) by introducing in the power transmission chain some degree of compliance, in the form of off-the-shelf springs or elastic and deformable components. Depending on where and how these compliant elements are inserted in the transmission, many different architectures of compliant actuators arise. The objective of this work is to analyze these different architectures from a theoretical point of view, derive new optimal design techniques and methodologies and, finally, apply these techniques to design new prototypes of robotic actuators devoted to specific applications.