Autonomous systems may lead to smarter and more efficient operations, but emerging risks are involved, because of lack of knowledge and operational experience with such systems, and challenges related to verification of safe performance. The objective of this paper is to clarify, categorize, and classify risk related to autonomous marine systems, and establish a foundation for risk management of such systems. Autonomous systems are usually associated with unmanned systems, but several manned systems, e.g., offshore oil and gas rigs and ships with complex automation and dynamic positioning (DP) systems, have certain control functionality that may be characterized as autonomous. Therefore, this paper addresses both manned and unmanned systems with different levels of autonomy. This means that the concept of autonomy in this paper includes a range of systems and operations with increasing complexity and major hazard potential, even though autonomous ships are used to exemplify. Hence, the paper addresses a broader approach in contrast to the traditional focus on robotics.