Unitary transformations in quantum field theory and bound states

Finding the eigenstates of the total Hamiltonian H or its diagonalization i s an important problem in quantum physics. However, in relativistic quantum field theory (RQFT), its complete and exact solution is possible for a few simple models only. Unitary transformations (UT's) considered in this surv ey do not diagonalize H, but convert H into a form which enables us to appr oximately find some H eigenstates. During the last years there have appeare d many papers devoted to the physical applications of such UT's. Our aim is to present is systematic and self-sufficient exposition of the UT method. The two general kinds of UT's are pointed out, as well as the distinct vari ations of them. We consider in detail the problem of finding the simplest H eigenstates for interacting mesons and nucleons using the so-called clothi ng' UT and Okubo's UT. These UTs allow us to suggest definite approaches to the problem of two-particle (deuteron-like) bound states in RQFT. The appr oaches are shown to yield the same two-nucleon quasipotentials in the first nonvanishing approximation. We demonstrate how the particle mass renormali zation can be fulfilled in the framework of the clothing procedure. Besides the UT of the Hamiltonian, we discuss the accompanying UT of the Lorentz b oost generators.