Presently, there exists significant potential to exploit active materials for dynamic control of mechanical behaviors, including adhesion and friction, to enable the design of systems with improved performance and new functionalities. Such systems can benefit a broad range of applications including the gripping and manipulation of objects in robotics and manufacturing, the temporary attachment of wearable devices, and the creation of tactile interfaces for virtual reality. The ability to control adhesion and friction on demand can be realized using materials with tunable stiffness. In particular, thermally responsive polymers, which can exhibit significant changes in Young’s modulus of two to three orders of magnitude, show great promise for maximizing adhesion and friction control. However, systems that rely on thermal actuation often suffer from slow heating and cooling, indicating a need for design strategies to expedite thermal response times. The work in this dissertation develops systems with tunable adhesion and friction via stiffness modulation of thermally responsive polymers. First, the high stiffness change of a thermally responsive thermoplastic composite is leveraged for adhesion tuning of a macroscale, soft end effector for versatile robotic grasping. Second, using a novel solvent-assisted molding technique, a shape memory polymer with thermally modulated stiffness is used to develop a high strength and tunable microstructured adhesive consisting of an array of composite microscale pillars, with the reduction to the microscale allowing for faster response times. Finally, the high stiffness change of a shape memory polymer is used to achieve contacts with tunable friction by transitioning from a low-friction Coulombic regime at high stiffness to a high-friction adhesion-dominated regime at low stiffness.