Biddy is a package for the manipulation of Boolean functions, sets and relations. It is based on Binary Decision Diagrams (BDDs), an efficient data structure. The results of the project feature a computer library (DLL for MS Windows, shared library for GNU/Linux), a toolkit for graphical representation of BDDs and improvements of algorithms and methods in the field of theoretical computer science. Biddy is an open source programme with a general GNU licence. It is widely usable, for example in designing logic circuits and in formal verification of communication protocols.