This paper presents the results of a systematic experimental campaign consisting of 13 symmetric punching tests on interior slab-column connections. The study focuses on the influence of varying the size of the supported area and the slenderness of the slab. Other investigated parameters are the flexural reinforcement ratio and the presence of shear reinforcement. The results of the present campaign and of previous tests are compared to the predictions of different codes of practice and to the critical shear crack theory (CSCT). The comparison shows that the CSCT and fib Model Code 2010 give the most consistent predictions, whereas the results of Eurocode 2 for small support sizes have a large scatter. The predictions of ACI 318 are observed as overall conservative in the investigated range. This study shows that slenderness has an important influence on the punching strength of slabs with shear reinforcement, despite the fact that it is neglected in many codes of practice.