Abstract: Protein plays an important role in the diet formulation of poultry to maintain/improve growth, feed utilization, immune functions and maximize productive performance, ensure bird welfare, as well as mitigate environmental risks and pollution by optimizing the use of this nutrient. Also, proteins assist in the biosynthesis of tissues and have many biological functions for growth and renewal of the body. Moreover, proteins exist in forms of hormones and enzymes which play crucial roles in the physiological processes in the body. Layer birds have low dietary Crude Protein (CP) requirements, thus identification of the optimum level of CP in layer diets, for either maximizing productive performance or economic returns of laying hens, requires more and more information/knowledge about birds needs of protein and its impacts on performance parameters and environmental pollution. This biomolecule plays active role in physiological dynamics, in stabilizing the immune barrier and to drive the forces for maintaining the enhanced production performance of layers. The broad goal of the current review is to highlight the importance of CP-diet and its positive and negative effects on body weight change, performance, feed and nutrient utilization, egg production parameters, egg quality criteria as well as chemical composition of egg and its fractions, egg shell quality, blood biochemical parameters, immune function, nutrient digestibility and nitrogen pollution. The discussed concepts and knowledge would be useful for both backyard poultry rearers and poultry industry holders by optimizing the level of crude protein in the fed diet and will definitely be helpful for gaining the economic profits and consumers satisfaction.