Hydraulic fracturing is widely used to create new fractures or extend and open existing ones. However, what exactly happens in the field is not well understood because, in most cases, only indirect information in form of pumping records, microseisms and the in-situ stress field are known. The MIT Rock Mechanics Group has developed and used a unique test equipment, with which the hydraulic fracture propagation can be visually observed while acoustic emissions are simultaneously recorded. All this can be done under different far field (external) stresses and different hydraulic pressures and flow rates. Interestingly, it is also possible to observe how the hydraulic fluid moves in the fractures. This allows one to relate details of the fracturing process to the micro-seismic observations and the boundary conditions thus providing the complete information that the field applications cannot. The testing equipment will be described first, followed by detailed descriptions of hydraulic fracturing experiments on granite and shale. These two rock types represent the typical usage of hydraulic fracturing: Granite for EGS (Engineered Geothermal Systems) and shale for hydrocarbon extraction.