Safety at sea is the protection from harm to people, property and the environment. Safety assurance in the case of autonomous sea going vessels is nontrivial due to the pace of change in enabling technologies and their disruptive impact. Historically accidents and incidents at sea have often been attributed to human error but the safety implications of a machine rather than a human making decisions whether fully or in part, is yet to be understood. Although the development of regulation of autonomy at sea is in its early stages, there is much activity to address safety of autonomy in maritime and elsewhere, along with a wealth of established safety practice from before its advent with good read across. Integral to the development of a safety case is the assessment and mitigation of risk i.e. the combination of frequency and severity of the consequence. When assessed quantitatively, frequency historically has been calculated using failure rates of components whose failure leads to the materialisation of the risk. These failure rates historically were derived from mechanical component failures, which were relatively easy to determine. Recently, the advent of software-controlled systems has introduced difficulties in defining failure rates; an issue which is exacerbated greatly in the context of autonomy, and within complex autonomous systems it is not possible to perform quantitative risk assessments using failure rates. This gives rise to a need to use different techniques to assess risk in the development of a maritime autonomy related safety case. Two recent and significant developments are the EU H2020 funded Autoship consortium aimed at demonstrating autonomy in short sea and inland waterway shipping and the European Maritime Safety Agency commissioned study into autonomous vessel safety risks and their assessment, SAFEMASS / RBAT. Meanwhile the umbrella body, Maritime UK is up to the seventh edition of its code of practice for industry players and the major ship classification societies have each published guidance documents in the intervening period. Two general purpose guidance documents are the UK’s Safety-Critical Systems Club “Safety Assurance Objectives for Autonomous Systems” and the “Safety Assurance of autonomous systems in Complex Environments (SACE)” from the Assuring Autonomy International Programme. Leaning on earlier established principles and practice, management of safety risk to a tolerable level and subsequent demonstration of safety case remain pivotal to safety assurance of maritime autonomy. Functional safety is the mitigating risks of system or component failures that would otherwise cause harm. Failure and hazard analysis techniques abound and adherence to standard IEC 61508 for electrical/control systems then facilitates the assigning of Safety Integrity Levels (SILs). Good practice may be read across from safety initiatives pertaining to self-driving road vehicles. Two that stand out are mitigating for functional insufficiencies or foreseeable misuse and so called “Safety Of The Intended Functionality” (SOTIF) and standard UL4600 for overarching safety case formulation. In this paper a summary of latest thinking and development related to safety of autonomy and relevant to sea going vessels is presented.