Principle purpose of this lecture is to explain on Database System Architectures. Here briefly discuss on Centralized Systems, Client–Server Systems, Transaction Server System and Data-server systems. Centralized database systems are those that run on a single computer system and do not interact with other computer systems. Here also focus on Computer System Architecture. Finally explain advantages and disadvantages of Distributed Database Management System, Data Fragmentation, Horizontal Fragmentation, Vertical Fragmentation etc.