Their extremely fruitful cooperation started in 1999. Since then, they have been involved in the conference series "Mathematics of Program Construction" and "Relational and Algebraic Methods in Computer Science", and others, which are right in the centre of their research interests. They have edited several proceedings for these series as well as special issues of the Journal of Logic and Algebraic Programming (later called Journal of Logical and Algebraic Methods in Programming), and Science of Computer Programming on these. The book is hence a coherent presentation of all this research.