Knowledge of the breakdown behavior of cheese is important for gaining insights into texture perception, flavor and nutrient release, as well as the origin of undesirable texture defects within the cheese matrix such as slits and cracks. In this novel study, changes in the microstructure of model Maasdam-style cheeses were observed in situ during tensile deformation by placing a microtensile stage directly under a confocal scanning laser microscope (CSLM), and recording force/displacement data simultaneously. A small indentation (called a notch) was made at a center point on the test specimens, and growth of the notch was observed under tensile deformation using CSLM. Widening of the notch, stretching of the protein network near the leading point of the notch, detachment of fat globules, and their subsequent release from the cheese matrix, as well as fracturing of the cheeses, partly along curd granule junctions, were all observed during tensile deformation. Moreover, an inherent micro-defect was observed at a curd granule junction within the cheese matrix and this micro-defect fractured along the curd granule junction under tensile deformation, suggesting that the micro-defects present within the cheese matrix could be a key underlying factor in the formation of undesirable slits or cracks. Further work showed that the fracture behavior of cheese was altered by changing ripening temperature, using different coagulant types, or by inhibition of residual chymosin. Such approaches could be applied to designing cheeses with specific texture profiles or for designing optimal cheese textures to withstand increased gas pressure during ripening in eye-type cheeses, which may help to prevent the formation of undesirable splits and cracks. Overall, this study demonstrated the potential of in situ imaging of cheese microstructure for developing a greater understanding of the breakdown behavior of cheese matrices.