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Today data is an important part of almost every modern application. However programming languages normally don't offer the same support for data manipulation as for program logic. It is obvious that there is a big gap between the two worlds; object oriented world and relational world. The consequences of the resultant gap are reflected in the fact that developers spend a lot of time with transferring data from one form (object) to another (relational) and vice versa. In this paper we will present the impedance mismatch between object-oriented programming languages and relational databases. Object-relational mapping will be presented as a technology that helps developers to overcome mentioned impedance mismatch with main focus on object-relational patterns and persistence ignorance. We will describe different object-relational mapping frameworks based on the .NET framework. On this base comparison criteria for evaluating used O-R mapping technologies will be defined. Defined criteria will then be used in comparison and analysis of presented O-R frameworks.