The light-front quantization of quantum field theories provides a useful alternative to ordinary equal-time quantization. In particular, it can lead to a relativistic description of bound systems in terms of quantum-mechanical wave functions. The quantization is based on the choice of light-front coordinates, where x + ≡ ≡ c t + z plays the role of time and the corresponding spatial coordinate is x − − ≡ ≡ c t − − z . Here, t is the ordinary time, z is one Cartesian coordinate, and c is the speed of light. The other two Cartesian coordinates, x and y , are untouched and often called transverse or perpendicular, denoted by symbols of the type x → → ⊥ ⊥ = ( x , y ) . The choice of the frame of reference where the time t and z -axis are defined can be left unspecified in an exactly soluble relativistic theory, but in practical calculations some choices may be more suitable than others.