Student ratings have been a controversial but important method for the improvement of teaching quality during the past several decades. Most universities rely on summative evaluations conducted at the end of a term or course. A formative approach in which each course unit is evaluated may be beneficial for students and teachers but has rarely been applied. This is most probably due to the time constraints associated with various procedures inherent in formative evaluation (numerous evaluations, high amounts of aggregated data, high administrative investment). In order to circumvent these disadvantages, we chose the Web 2.0 Internet application Twitter as evaluation tool and tested whether it is useful for the implementation of a formative evaluation. After a first pilot and subsequent experimental study, the following conclusions were drawn: First, the formative evaluation did not come to the same results as the summative evaluation at the end of term, suggesting that formative evaluations tap into different aspects of course evaluation than summative evaluations do. Second, the results from an offline (i.e., paper-pencil) summative evaluation were identical with those from an online summative evaluation of the same course conducted a week later. Third, the formative evaluation did not influence the ratings of the summative evaluation at the end of the term. All in all, we can conclude that Twitter is a useful tool for evaluating a course formatively (i.e., on a weekly basis). Because of Twitter's simple use and the electronic handling of data, the administrative effort remains small.