  In this contribution, I will give a brief survey of present techniques to
treat the bound state problem in relativistic quantum field theories. In
particular, I will discuss the Bethe-Salpeter equation, various quasi-potential
equations, the Feynman-Schwinger representation, and similarity transformation
methods for Hamiltonian approaches in light-front quantization. Finally, I will
comment on a related similarity transformation in the usual equal-time
quantized theory.
