An overview of the causes and effects of porosity in rubber has been presented. The formation of porosity in rubber can be attributed to the heterogeneous nucleation and diffusion controlled bubble growth. The process proceeds simultaneously with vulcanization, so bubble formation in viscous fluids and in crosslinked rubber has been used to elucidate the mechanism. Porosity is caused by the supersaturation of rubber with volatiles and by insufficient external pressure during vulcanization. Some of the sources of these volatiles have been described. The effects of compounding, mixing and processing on porosity have been covered, and various methods of detecting and preventing porosity have been reviewed. Finally, the effects of porosity on the mechanical properties of rubber have been described.