Preface The aim of the Transformation Tool Contest (TTC) series is to compare the expres- siveness, the usability, and the performance of transformation tools along a number of selected case studies. A deeper understanding of the relative merits of different tool features will help to further improve transformation tools and to indicate open problems. This contest was the twelfth of its kind. For the seventh time, the contest was part of the Software Technologies: Applications and Foundations (STAF) federation of confer- ences. Teams from the major international players in transformation tool development have participated in an online setting as well as in a face-to-face workshop. The previous editions have focused on cases exploring specific research directions in model-transformation (such as bi-directional transformation, traceability, verifiability or performance). This year the committee selected a case study that presents a well-known simple transformation, namely the Truth Tables to Binary Decision Diagrams. The aim of this case is to serve as a showcase of the various directions that model transformation research is going towards at the moment. Similarly to previous years, there was also a live case study. These proceedings comprise descriptions of the case study and all of the accepted solutions. In addition to the solution descriptions contained in these proceedings, the implementation of each solution (tool, project files, documentation) is made available in public version control repositories. TTC 2019 involved open (i.e., non anonymous) peer reviews in a first round. The purpose of this round of reviewing was that the participants gained as much insight into the competitors’ solutions as possible and also to identify potential problems. At the workshop, the solutions were presented. The expert audience judged the solutions along a number of case-specific categories, and prizes were awarded to the highest scoring solutions in each category. Finally, the solutions appearing in these proceedings were selected by our programme committee via single blind reviews. The full results of the contest are published on our website1 . Besides the presentations of the submitted solutions, the workshop also comprised a live contest that was announced to all STAF attendees and participants were given four days to design, implement and test their solutions. Some of the live case solutions were also submitted for a peer review and the accepted ones are included in the second part of this proceedings. The contest organisers thank all authors for submitting cases and solutions, the contest participants, the STAF local organisation team, the STAF general chair Mark van den Brand, and the program committee for their support. 19th July, 2019 Antonio Garcia-Dominguez Eindhoven, The Netherland Georg Hinkel Filip Křikava 1 http://www.transformation-tool-contest.eu/ Organisation The Transformation Tool Contest has been organized by TUE in Eindhoven, The Nether- land. Program Committee Konstantinos Barmpis University of York, United Kingdom Juan Boubeta-Puig University of Cádiz, Spain Erwan Bousse Vienna University of Technology, Austria Gwendal Daniel AtlanMod - Inria, France Antonio Garcia-Dominguez Aston University, United Kingdom Georg Hinkel FZI Research Center of Information Technology, Ger- many Tassilo Horn SHD, Germany Akos Horvath Budapest University of Technology and Economics, Hun- gary Filip Křikava Czech Technical University, Czech Republic Arend Rensink University of Twente, The Netherlands Massimo Tisi Ecole des Mines de Nantes, France Gergely Varro Technische Universitat Darmstadt, Germany Ran Wei University of York, United Kingdom Copyright c 2019 for the individual papers by the papers’ authors. Copyright c 2019 for the volume as a collection by its editors. This volume and its papers are published under the Creative Commons License Attribution 4.0 International (CC BY 4.0).