  Testing of autonomous systems is extremely important as many of them are both
safety-critical and security-critical. The architecture and mechanism of such
systems are fundamentally different from traditional control software, which
appears to operate in more structured environments and are explicitly
instructed according to the system design and implementation. To gain a better
understanding of autonomous systems practice and facilitate research on testing
of such systems, we conducted an exploratory study by synthesizing academic
literature with a focus group discussion and interviews with industry
practitioners. Based on thematic analysis of the data, we provide a
conceptualization of autonomous systems, classifications of challenges and
current practices as well as of available techniques and approaches for testing
of autonomous systems. Our findings also indicate that more research efforts
are required for testing of autonomous systems to improve both the quality and
safety aspects of such systems.
