In the modern age of miniaturisation, Smart Materials, a type of material that reacts mechanically to a certain stimulus, have become an integral part of this revolution. Among these materials, Shape Memory Alloys (SMAs), which have the highest volumetric work density, are the ideal candidate in creating lightweight and miniature actuators. These alloys, after being deformed, are able to revert back to their original shape when exposed to heat. This exotic behaviour has allowed them to be the core active component in a plethora of applications such as grippers, bio-mimetic robots and surgical instruments. Despite their high work density, their implementation comes with some challenges. While the Shape Memory Effect (SME), the ability to recover strain when a thermal load is applied, is a remarkable behaviour, it is also a complex and multi-physical one. This complicates their design and makes it difficult to predict their behaviour. Furthermore, these alloys are only able to recover strain when deformed at low temperatures. This implies that a biasing element is required to exploit these materials in reversible actuators. Due to these limitations, the work density of SMAs, when implemented as actuators in robotic systems, are often much lower than their theoretical maximum. In this thesis, the various types of SMA actuator implementations from different applications are examined to understand the design requirements and subsystems that are necessary to build an actuator. A holistic approach is, then, used to construct a design methodology to create highly integrated actuators in the hopes of preventing the work density degradation present in traditional SMA-based systems. Here, in this work, the identified subsystems of the SMA actuator are combined to serve as a multi-functional element in the novel integrated actuator. The work employs different strategies to integrate the SMA actuator into robotic systems while also proposing adapted sizing methodologies. The resulting SMA-powered system can be sized to be lightweight, compact and dynamic. Various case studies are presented that utilise the proposed holistic design approach and sizing methodologies to serve as a proof-of-concept and to validate the methodology. In this work, compliant and flexure-based mechanisms are exploited to exclude the need for a dedicated biasing element and create lightweight SMA-powered grippers to demonstrate the advantages of the proposed methodology. Additionally, utilising the mechanical behaviour of the SME, a lightweight mechanically-controlled crawling robot is designed and implemented. Furthermore, with the help of topology optimization and kirigami-inspired design, the work details the creation of compliant SMA structures that allow the material to generate multiple outputs while remaining compact and easy to assemble. Lastly, a novel bistable gripper is designed and sized to experimentally validate the design methodologies proposed in this work. The work demonstrates the different areas in which the degradation of the work density occur in traditional SMA systems. In this regard, design methodologies accompanied with sizing strategies are proposed that allows the creation of lightweight, high bandwidth and integrated SMA-based robotic systems. The results in this thesis, reveal the extraordinary value of SMAs in creating lightweight robotic systems and presents various strategies to allow the further integration of the alloy within the system.