Introduction of physics behind diesel spray and its combustion is the purpose of this monograph. The main subjects of here are diesel spray penetration and air entrainment. These subjects have strong relationships, and momentum conservation of diesel spray is the physics behind these relationships. Effective heat release control is needed for diesel combustion control, and air entrainment is one of the physical controlling factors of diesel combustion. It means that physics concerning diesel spray should be understand more deeply. Author's opinions and some proposals are included in this monograph. Author believes that these opinions and proposals will help the advanced research work of diesel engine combustion.