Acknowledgements
We would like to thank David Muller for giving us the opportunity to put together this special edition. In addition, we would also like to thank the ad hoc reviewers Sarah Astill, Karen Birch, Andy Levy, Neil Messenger, Marwa Nasr, David Sugden, Bert Steenbergen, Phil Vickerman, Daniella Strauss, and Andrew Wilson for their constructive reviews of the papers included.