  Accountability is an often called for property of technical systems. It is a
requirement for algorithmic decision systems, autonomous cyber-physical
systems, and for software systems in general. As a concept, accountability goes
back to the early history of Liberalism and is suggested as a tool to limit the
use of power. This long history has also given us many, often slightly
differing, definitions of accountability. The problem that software developers
now face is to understand what accountability means for their systems and how
to reflect it in a system's design. To enable the rigorous study of
accountability in a system, we need models that are suitable for capturing such
a varied concept. In this paper, we present a method to express and compare
different definitions of accountability using Structural Causal Models. We show
how these models can be used to evaluate a system's design and present a small
use case based on an autonomous car.
