  Simulation is an integral part in the process of developing autonomous
vehicles and advantageous for training, validation, and verification of driving
functions. Even though simulations come with a series of benefits compared to
real-world experiments, various challenges still prevent virtual testing from
entirely replacing physical test-drives. Our work provides an overview of these
challenges with regard to different aspects and types of simulation and
subsumes current trends to overcome them. We cover aspects around perception-,
behavior- and content-realism as well as general hurdles in the domain of
simulation. Among others, we observe a trend of data-driven, generative
approaches and high-fidelity data synthesis to increasingly replace model-based
simulation.
