This book summarizes the research findings as an outcome from the IUTAM Symposium 'Mechanics and Reliability of Actuating Materials held successfully on September 1-3, 2004 at Tsinghua University, Beijing, China. Actuating materials hold a promise for fast-spreading applications in smart structures and active control systems, and have attracted extensive attention from scientists of both mechanics and materials sciences communities. High performance and stability of actuating materials and structures play a decisive role in their successive applications as sensors and actuators in structural control and robotics. Toward this end, scientific efforts are of paramount significance to gain a deep insight into the intricate deformation and failure behaviors of actuating materials. Examples worthy of intensive exploration are: (1) the constitutive relations of actuating materials that couple mechanical, electrical, thermal and magnetic properties, as well as incorporate phase transformation and domain switch; (2) the physical mechanisms of deformation, damage, and fatigue crack growth of actuating materials; (3) the development of failure-resilient approaches that base on the macro-, meso-, and micro-mechanics analyses; (4) the investigation of microstructural evolution, stability of phase transformation, and size effects of ferroelectric ceramics, shape memory alloys and actuating polymers. The above problems represent an exciting challenge and form a research thrust of both materials science and solid mechanics.