Differential-algebraic equations (DAEs) are the most natural way to mathematically model many complex systems in science and engineering. This book provides a guide to the theory and practice of modelling with DAEs. In particular, the reader will learn to maximise the performance of their models by optimising the design parameters. Presented within are cutting-edge theory and state-of-the-art numerical methods for the optimal control of differential-algebraic equations, alongside real-world applications of the results. This accessible treatment of the subject, written by leading experts, is suitable for applied mathematicians, engineers and computational scientists from a variety of disciplines. It will be of interest to those developing theory and those working on real-world applications, especially in the optimal control of problems in chemical and mechanical engineering.