In view of the fact that really observable particles are not bare but dressed with their self field, the procedures to treat rigorously the dressed particles are presented. For this purpose, the Green function formalism by Schwinger and covariant equations by Bethe and Salpeter are useful. In this paper, we first discuss the relations among various covariant formulations. Then it is investigated how to normalize the Beteh-Salpeter wave functions and how to express the expectation values of given observables in terms of Bethe-Salpeter wave functions. Finally the difference of treatments between dressed and bare particles are shown in connection with the scattering problems.