I. INTRODUCTION Autonomous systems have a long history in the fields of Artificial Intelligence (AI) and Robotics. However, only through recent advances in technology has it been possible to create autonomous systems capable of operating in longterm, real-world scenarios. Examples include autonomous robots that operate outdoors on land, in air, water, and space; and indoors in offices, care homes, and factories. Designing, developing, and maintaining intelligent autonomous systems that operate in real-world environments over long periods of time, i.e. weeks, months, or years, poses many challenges. This special issue focuses on such challenges and on ways to overcome them using methods from AI. Long-term autonomy can be viewed as both a challenge and an opportunity. The challenge of long-term autonomy requires system designers to ensure that an autonomous system can continue operating successfully according to its real-world application demands in unstructured and semistructured environments. This means addressing issues related to hardware and software robustness (e.g., gluing in screws and profiling for memory leaks), as well as ensuring that all modules and functions of the system can deal with the variation in the environment and tasks that is expected to occur over its operating time. Early research in longterm autonomy for mobile robots focussed extensively on the problem of coping with environment variation, e.g., performing visual localisation over seasonal changes, or SLAM in a dynamic environment. Once such challenges are overcome, the long-term operation of an autonomous system provides the opportunity to specialise that system to its tasks and operating environment, potentially improving both its general robustness and more specific task performance. This specialisation may come through the use of extensive system logs to detect and fix bugs, or through automatic online adaptation via machine learning. There is also the opportunity to aid development through the use of logs to create tests and simulations which are representative of a specific system deployment. A great many research fields have the potential to contribute to enabling and improving long-term operation of autonomous robots. For example, formal methods can be used to verify robot software or control policies to ensure safe long-term operation, and novel locomotion methods can be used to minimise a robot's long-term energy usage. However, we chose to focus on AI techniques for long-term