This volume covers several aspects of the Acarology as presented at The Fourth Symposium of the European Acarologists held in Siena, Italy, July 24-28, 2000. The meeting addressed the direction of research, as well as highlighted the challenges of this scientific field. On the basis of the presentations and invited papers, there is evidence of a "new" Acarology based on modern techniques and methods of investigations. It also demonstrated the importance of the "classical" Acarology, based on the alpha taxonomy and original studies. The proceedings include selected original works on taxonomy; systematics; phynology; molecular biology; genetics; ecology (relationships between Acari and their plant/animal host); life strategies; morphology (reproductive anatomy; sperm transfer; structure and functioning of various organs of Acari); physiology; and control and behaviour of mites and ticks.