Abstract
Several authors and organisations have over the last 40 years suggested various ways to describe different levels of automation. In this paper we give an introduction to some of the most relevant models and present some of the challenges faced when using them. Furthermore, we suggest a new definition for autonomy, based on responsibility instead of functionality. The paper analyses some relevant cases based on the suggested definition of autonomy, and points towards some issues that should be investigated in the maritime domain related to current definitions for autonomous ships.