The present study drives at exposing the importance of the informal economy in supporting and developing the formal economy and consequently, pushing the wheel of both sustainable and economic development plans alike. Some estimations indicate that the size of the informal sector in the economies of developing countries represents a high percentage ranging between 30-70% of the gross national product of the states; which means that a large proportion of economic activities are to be operated with less efficiency and less effectiveness than the demanded standards. Added to that, the means of integrate this informal economy within the formal one; highlighting also the positive and negative aspects of the informal economy and its role in sustainable development plans and extrapolating the religious and legislative aspect in the legitimacy of profits and business of the informal economy. Classical people believe that the capitalist system can achieve full employment of 100% of employment and that any defect in the system the economic situation due to wars or demonstrations will be automatically adjusted in the case of full employment. Several states of the world suffer from a specific phenomenon that threatens their economics, being represented in the informal economy . this type of economics cannot be underestimated; since it represents about 40% of the total local product. This is because of the acceleration of its development rates. The problem is crystallized in that the data and information that reflect the Egyptian economy's status, are inaccurate, because of Egypt's loss of a great portion of its effective economy,  namely, the informal economy which is not registered for tax and doesn't exist under the umbrella of the state's legislations and laws. Consequently, the data and statistics of the state are inaccurate. The researchers count on two integrated methods in preparing the current study, the descriptive inductive method and the deductive method, as it is necessary to connect the intellectual framework to the practical reality which demonstrates its applicability in the study framework.