This book chapter reviews fundamental concepts and results in the area of online algorithms. We first address classical online problems and then study various applications of current interest. Online algorithms represent a theoretical framework for studying problems in interactive computing. They model, in particular, that the input in an interactive system does not arrive as a batch but as a sequence of input portions and that the system must react in response to each incoming portion. Moreover, they take into account that at any point in time future input is unknown. As the name suggests, online algorithms consider the algorithmic aspects of interactive systems: We wish to design strategies that always compute good output and keep a given system in good state. No assumptions are made about the input stream. The input can even be generated by an adversary that creates new input portions based on the system’s reactions to previous ones. We seek algorithms that have a provably good performance. Online Algorithms