In recent years, considerable electricity demand in many countries associated with the high development in various sectors including industrial, tourism, commercial and residential loads, requires power system expansions at all levels, generation, transmission and distribution. In Egypt, for example, three large generating power plants of 4800 MW each have been recently installed ^[1] and ^[2]. Also, a large photovoltaic power plant of 1800 MW was constructed at Benban in Aswan ^[3], in addition to the planned 4800 MW nuclear power station at El-Dabaa and expected increase of renewable energy resources such as wind and solar. This paper describes the main security and reliability criteria for power system planning ^[4].  The objective is to provide the basic information on the technical design specifications and criteria, technical terms and equipment parameters for successful planning and the requirements of normal case (N), contingency cases (N-1) ^[5-9]. Additional criteria, that covering aspects of quality of supply which are often contained in the grid code ^[5] and ^[6], should be considered in conjunction with the security standards ^[7-9]. The paper describes the main features, criteria and technical design requirements in the to cope with the development of power grids including connection of large generating plants, transmission system expansion, substation expansions and international connections to external power systems