The history of squeeze casting as a fabrication process for MMCs is briefly described in this chapter. Squeeze casting is one of the liquid state fabrication techniques. We can learn essentials of the fabrication of composites from studying squeeze casting, because, during squeeze casting, mechanical energy is converted into interface energy at the reinforcement/matrix interface. This energy conversion is economical and efficient and means that composites can be fabricated with minimum energy using squeeze casting. In this chapter, the threshold pressure equation for infiltration into preforms is introduced, and the infiltration energy, effect of preform preheat temperature, and microscopic phenomena occurring during infiltration of molten metal are discussed theoretically.