This volume is an outcome of the REFT 2005 Workshop on Rigorous Engineering of Fault-Tolerant Systems held in conjunction with the Formal Methods 2005 conference at Newcastle upon Tyne, UK, in July 2005. The authors of the best workshop papers were asked to enhance and expand their work and a number of well-established researchers working in the area contributed invited chapters. From the 19 refereed and revised papers presented, 12 are versions reworked from the workshop; 9 of them are totally new. The book is rounded off by two provocatively different position on the role of programming languages.