PhD defense in germany: how do they decide for the final grade? I am a PhD student in Germany (physics) and I will have my PhD defense in some months. I am not German. 
My thesis is mainly based on 2 PRL's and a paper not yet published (submitted to PRB). The reviews (Gutachten) on my thesis are positive: I've got 1.3 and 1.7 (1 is very good, 4 is sufficient). 
The defense will consist of an oral presentation of my thesis and of an oral exam (Rigorosum) in the presence of two professors (two main topics, one of which is completely not related with my thesis). 

My question is, how much are important the presentation, the rigorosum, and the reviews respectively, to the decision of the final grade? Is it possible to fail the PhD defense, having good reviews? Actually, I am mainly concerned about nasty questions after the thesis presentation and during the Rigorosum...
To me it seems that your defense will decide whether you will get the grade 1.3 or 1.7. You will certainly not fail. 

During my PhD it was roughly 2/3 the written thesis and 1/3 the defense, which is divided equally in your presentation and the rigorosum. However, for you I think this is not relevant since your Gutachten gave different grades and your performance will decide whether Gutachten 1 or 2 is "right". 

If I were you I would take the defense seriously and be prepared for the topic the professors are going to ask you. Often professors give some hint on the topic or might even tell you in advance. 

You should know that 1.7 will be rounded to 2, so I think it will be a "cum laude" vs. "magna cum laude". So it makes a difference. 

To my knowledge virtually all grades are summa cum laude (1+) (approx 20%-30%), magna cum laude (1) and cum laude (2). So cum laude is practically the lowest grade, although it is a 2.

BTW: For the summa cum laude (the distinction) you need 1.0 in both Gutachten and also make an excellent presentation, where professors test you for your limits. Professors often use it to indicate that they think you are ready for the academic career. Also during selection of some programs of the DFG it is expected to have such a grade.

I hope this does answer your question.