Preface : The target of the special issue was to receive submissions contributing to the hot topics on formal approaches for active and passive testing of distributed and networked systems. With this aim and after a careful selection, four papers have being chosen to be published in this special issue. In addition, the special issue includes a survey on active and passive testing as well as on testing (in) the cloud. The editors of the special issue would like to thank the members of the guest editorial board for their help during the reviewing process: Rui Abreu, Khaled El-Fakih, Toru Hasegawa, Thierry Jéron, Pascale Le Gall, David Lee, Stephane Maag, Eliane Martins, Inmaculada Medina-Bulo, Mercedes G. Merayo, Adenilso Simao, Kenji Suzuki, Jan Tretmans, Farn Wang, Carsten Weise, Franz Wotawa, Nina Yevtushenko, and Fatiha Zaïdi. We sincerely hope that the subscribers of the journal enjoy reading the papers contained in this special issue and that these contributions are useful for their research