Dear friends and colleagues, faithful and new readers, I am so happy to greet you again. This is a very special issue for two reasons: first, it addresses the topic that is extremely important and ‘hot’ today, and second, because it is issue No.15 (!) Frankly, it is hard to believe that exactly five years ago, in May 2009 we started this journal and could not even imagine what the future would bring us. Who could predict that we would gain readers from six continents and 129 countries, or that 77% of our readers would return to our website for almost every new journal issue? Pleasant surprises all over – so many sophisticated authors, wonderful papers, and so many great readers.