In this survey, we have concluded that carbon black reinforces ultimate properties of rubber by tear deviation. This can occur at the colloidal level, as the tear is forced to pass around carbon black aggregates, thus increasing the area of the torn surface. Colloidal tear deviation may be the cause of the increase in threshold tearing energy which appears to be a small but significant component of reinforcement. Most reinforcement under ordinary test conditions is due to macroscopic tear deviation. This appears to be caused by strands of carbon black aggregates which force the growing tear to change direction. Macroscopic tear deviation is readily observed in a trouser tear test as stick-slip or knotty tear. In the tensile strength test, tear deviation prevents catastrophic rupture until a high stress is reached. In order to be effective in this or other ultimate properties, the carbon black must not debond; i.e., failure must go through the rubber rather than through the debonded rubber—filler interface. Reinforcing carbon blacks do not debond (from diene rubbers) under normal conditions and so do not require further strengthening of the rubber—filler bonds.