In the beginning, file system was used to handle the database. In this system each application (software that use and works on that data) defines and defined and maintained its own data .thus data was not independent of these applications i.e. In order to change the data definition we also had to change the application too. But now database management systems are used to manage the databases. These systems keeps the data independent of applications and each application can access the data through the same interface of database management systems.Application programs are immune to changes in logical or physical organization of data and vice versa. As Data can be defined at two levels, either logically or physically. The logical data definition is known as schema definition whereas the latter is known as physical data description. • Logical data independence refers to the immunity of application programs to the logical structure of database. For example if we add more attributes to a relation in database, the data access methods used by application should not be affected. • Physical data independence says that the application programs should not be concerned with the physical data organization. Therefore the application programs should not need to be modified when physical data organization is changed.