Wikipedia
The Dr.-Ing. (acronym for Doktor-Ingenieur or Doktor der Ingenieurwissenschaften) is the German engineering doctorate degree, comparable to the Doctor of Engineering, Engineering Doctorate, Doctor of Science (Engineering), Doctor of Science (Technology) or a PhD in Engineering or Architecture.
It was first introduced in 1899 at the Prussian Institutes of Technology, including what is now known as RWTH Aachen. The other German states adopted it in the following years. In contrast to the other historic doctoral degrees (e.g. Dr. phil., Dr. jur. or Dr. med.), the Doktor-Ingenieur was not titled in Latin but German, and therefore written with dash (Dr.-Ing.).
In the field of computer science, some universities offer the choice between Dr.-Ing. and Dr. rer. nat. based on the primary focus of the dissertation. If the contributions focus slightly more on applied scientific engineering a Dr.-Ing. is given, while a Dr. rer. nat. is preferred if the dissertation contains more theoretical scientific contributions. It may be worth noting that a German doctorate is usually a research doctorate and is awarded in the context of the so-called promotion that also requires a dissertation.
It should not be confused with a Dutch double title dr. ing., indicating one holds a doctorate (Dr.) as well an engineer's degree (Ing.) from a Dutch polytechnic (i.e. Hogeschool).