We study the class of almost uniserial rings as a straightforward common generalizationof left uniserial rings and left principal ideal domains. A module is almost uniserial if any two of its submodules are either comparable in inclusion or isomorphic. And a module is almost serial if it is a direct sum of almost uniserial modules. We give some properties of almost uniserial rings and modules. We also consider Artinian commutative rings which are almost uniserial and obtain a structure theorem for these rings. In the sequel we obtain some results which are iired by a characterization of Artinian serial rings as rings having all left (or right) modules serial. We prove that if $ R $ is a local ring and all left R-modules are almost serial then $ R $ is an Artinian ring which is uniserial either on the left or on the right. Also a connection between local rings having all left and right modules almost serial, local balanced rings studied by Dlab and Ringel and local K?the rings have been produced. We also prove Morita invariance of the almost serial property and list some consequences. In continuing the study of decomposition aspects, one natural question is that if we weaken the assumption of almost seriality of all modules and just assume that each ideal is almost serial what will be the structure of the ring. This question is completely answered for commutative rings.