One of the little-studied problems of modern mechanics and the physics of fracture is the crack branching in materials of a different nature. For its research, an analysis of the criteria and mechanisms for crack branching. The investigations of crack branching in polymers and metals, fractographic investigation of the fracture surface, criteria for crack branching as a dynamic stress intensity factor, crack speed are considered. Experimental studies of crack branching in a brittle polymer, measuring of crack velocity by breaking the conductive strips with rectilinear propagation and branching of the crack have been carried out. A measurement installation was created on the basis of a precision signal converter of resistance thermometers and thermocouples “Tercon” connected to a computer. It is established that the crack at branching in brittle polymers reaches the limiting propagation speed 500–800 m/s. A hypothesis is proposed for the physical mechanism of crack branching.