Aggregate interlocking is considered as one of the most important shear transfer actions in cracked concrete members without shear reinforcement. This research project analyses the fundamentals of aggregate interlocking by means of a specific experimental programme. Tests are carried out by precracking notched concrete specimens in pure tension and imposing mixed-mode kinematics (opening and sliding) thereafter. The tests are performed with varying opening and sliding ratios and on various concrete types. The development of cracking is tracked using Digital Image Correlation, which allows also to investigate secondary cracks developing during the mixed-mode phase. The various test results are presented and discussed. They are eventually compared with further tests and analytical models from the literature.