The third edition of Nonlinear Optics uses the SI system of units, so let us look at that system first. In the SI system of units the vacuum is treated as a material medium of permittivity and permeability . Therefore, the factors and appear in the equations. Also there is the factor of . In the SI system, distance is measured in meters (m), mass in kilograms (kg), and time in seconds (s). The meter is defined as the distance that light travels in a given interval of time. The second is defined by 9,192,631,770 oscillations between the two hyperfine levels of the ground state of a atom at rest at a temperature of 0 K. The newton (N) is the unit of force, which is kg m/. The joule (J) is the unit of energy, which is kg . The coulomb (C) is the unit of electrical charge, defined such that the force between two charged particles, each of 1 C of charge and separated by 1 m, is 1 N. The unit of electrical current is the ampere (A), which is 1 C/s. Finally, the volt is the unit of electrical potential, which is 1 J/C. With these definitions, the author wrote the Maxwell equations using the SI units of measure. The units and the names of the field vectors are then given: electric field, electric displacement, magnetic field or induction, magnetic intensity, polarization, and magnetization. The constitutive relations give the relations between the electromagnetic field vectors and the properties of materials. The author derives Poynting's theorem (the Poynting vector is the rate at which electromagnetic energy passes across a unit area that is normal to the direction of propagation) from Maxwell's equations.