Dynamical behaviors of an exciton interacting with phonon and photon fields are discussed for the typical cases — weak and strong limits of these interactions — with simplifying assumptions on the electronic structures of an exciton. After reviewing the general features of the motion of an exciton in phonon field (§2), we discuss how these features manifest themselves in the line-shape of exciton absorption band (§3). A tentative explanation of the Urbach rule on the tail part of fundamental absorption is presented (§4). As for the exciton-photon interaction, we give a brief survey of Hopfield's work, with particular emphasis on the desirability of general absorption and dispersion formulae for the case where neither exciton-photon interaction nor exciton-phonon (or any other dissipative) interaction is small (§5). Then we apply his result to (direct) radiative annihilation of an exciton, and the second order perturbation theory to indirect radiative annihilation (§6). Dissociation of an exciton in phonon field (§7) as well as non-radiative recombination (into an exciton) and radiative annihilation of a free electron-hole pair (§8) is also discussed. Summarizing these results, a brief consideration is given on the photoconductivity and luminescence of an exciton (§9).