In this thesis we consider some problems and also generalize some results related to indigent modules and subinjectivity domains. We prove that subinjectivity domain of any right module is closed under factor modules if and only if the ring is right hereditary. Indigent modules are the modules whose subinjectivity domain is as small as possible, namely the modules whose subinjectivity domain is exactly the class of injective modules. We give a complete characterization of indigent modules over commutative hereditary Noetherian rings. The commutative rings whose simple modules are injective or indigent are fully determined. The rings whose cyclic right modules are indigent are shown to be semisimple Artinian. We also give a characterization of t.i.b.s. modules over Dedekind domains.