Step I: Ideally before or at the beginning of your doctoral studies, apply for admission as a doctoral candidate at the Faculty of Mathematics and Natural Sciences
Follow the instructions of the Doctoral Office of the FMN

Step II: Within three months after starting your doctoral studies, register with the GSfBS by submitting your supervision agreement

Each doctoral candidate assembles a thesis advisory committee (TAC) consisting of the PhD advisor and two further scientists independent of the advisor, the mentors. We recommend that one mentor is from a field closely related to the doctoral research project and the other from a different field but with complementary expertise. One of the mentors can be an independent group leader.

The TAC composition is announced to the graduate school administration in the context of the supervision agreement: Please check the information provided below, download the form and open it with Adobe Acrobat Reader for electronic signatures.
 

At the Unversity of Cologne supervision agreements are mandatory for Doctoral Candidates. 
These agreements include a section to enter the TAC members. 

Doctoral Candidates, who apply for membership with the GSfBS, need to use this form for a preparatory conversation with their PhD advisor before filling and signing the supervision agreement. 
Within three months upon commencing the PhD work, filled and signed agreements are send to 

isabell.witt(at)uni-koeln.de           (last names starting with A - L) or 
katerina.vlantis(at)uni-koeln.de  (last names starting with M - Z)

The GSfBS only accepts the supervision agreement provided above, 
the respective supervision agreement of the Faculty of Mathematics and Natural Sciences 
or agreements from a number of specialised PhD programmes at the Faculty.