In the early stage of building a new informational system requirement engineering involves all the processes related to the requirement system and research of problems emerged during this process. In this stage the requirements specification of corresponding information system are built. Many different notations serve for this purpose, selection of which is dependant on available resources and project goals. Requirements specification must be of good quality. It is evaluated by characteristics of quality. Specifications could be analyzed and improved by using various tools including CASE tools. The specification is developed and its quality is improved by making analysis of requirements. Well done specification is basis for creating high leveled functionality system.