The tutorial presents the state-of-the-art in methodologies and tools for round-trip-engineering of object-oriented software systems. This covers not only (UML) class diagrams but also behavior diagrams like message sequence charts, collaboration diagrams, state charts, and activity diagrams. In addition, round-trip engineering with standard design patterns is addressed. This covers design by combining design patterns, implementation of design patterns (including code generation), and the recognition of standard design patterns in code fragments. First, we present the state-of-the-art in deriving semi-automatically implementations (in Java, C++ or the like) from design documents. Second, the tutorial will show the state-of- the-art in the translation of UML behavior diagrams to Java or C++ code. Third, design patterns provide additional information and means for the derivation of a valid implementation. Finally, the tutorial adresses recognition of occurrences of design patterns in code fragments for re-engineering and round-trip purposes.