Finding the eigenstates of the total Hamiltonian H or its diagonalization is the important problem of 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 survey do not diagonalize H, but convert H into a form which enables us to find approximately some H eigenstates. During the last years there have appeared many papers devoted to physical applications of such UT's. Our aim is to present a systematic and self-sufficient exposition of the UT method. The two general kinds of UT's are pointed out, distinct variations of each kind being possible. We consider in detail the problem of finding the simplest H eigenstates for interacting mesons and nucleons using the so-called ``clothing'' UT and Okubo's UT. These UT's allow us to suggest definite approaches to the problem of two-particle (deuteron-like) bound states in RQFT. The approaches are shown to yield the same two-nucleon quasipotentials in the first nonvanishing approximation. We demonstrate how the particle mass renormalization can be fulfilled in the framework of the ``clothing'' procedure. Besides the UT of the Hamiltonian we discuss the accompanying UT of the Lorentz boost generators.