Abstract
In this chapter, the definition of Mission Critical Systems, users, and services will be introduced, then main applications of such system will be listed, the general requirements will be investigated, and finally a distinction between different types of Mission Critical Systems will be briefly discussed. Nowadays, many governmental and non-governmental organizations (NGOs) play critical roles in its operations on everyday basis. The services provided by such organization is of special nature that makes it less tolerant to executional or operational errors; such services are referred as Mission Critical Services. The criticality of the service implies that it has a set of special requirements that distinguish it from other services, and it should be available anytime, anywhere within the service operational scope, and also it is supposed to be functioning within expectation limits regardless of the operational circumstances or running conditions.