This chapter presents experiences with modelling activities at the University of Hamburg, in which small groups of students from upper secondary level intensely work for 1 week on selected modelling problems, while their work is supported by pre-service-teachers. The paper presents one authentic solution of a group of students concerning a biological question and describes the approach of the students in detail. The form of an authentic description also includes mathematical errors and thoughts of the students that either have been discounted during the development of a solution or found their way into their ultimate solution. So an insight into the modelling-activities of the students during the modelling week can be gained. Finally some results of an evaluation are presented that has been conducted after the modelling week.