Nitrogen is an essential limiting resource for plant growth, and its effective utilization can increase the adaptability of plants. This is because nitrogen is required for the regulation of more than a hundred genes in plants and influences their physiological processes. Leaf nitrogen allocation is defined as the proportion of nitrogen distributed in every plant leaf cell and free compounds. Leaf nitrogen allocation not only determines the photosynthetic capacity of leaves but also affects their toughness and chemical defenses. Therefore, research on nitrogen allocation in leaves is of great importance. In this study, we evaluated the mode of leaf nitrogen allocation. Leaf nitrogen is generally distributed in the cell wall, cell membrane, nucleus, and organelles such as chloroplasts and mitochondria in the cytoplasm; leaf nitrogen is also found in free compounds. Usually more than half (up to 75%) of the leaf nitrogen is assigned to chloroplasts, which are involved in photosynthesis. An increase in the amount of nitrogen involved in photosynthesis results in faster plant growth. The amount of leaf nitrogen allocated to cell walls can indirectly result in blade tenacity, which is a basic method of plant self-defense; 10%-30% of the nitrogen is assigned to the cell walls. Leaf nitrogen is mainly found in the form of plant proteins, and usually 71%-77% of the leaf nitrogen is found in the form of proteins, followed by free amino acids, alkaloids, cyanogenetic glycosides, phospholipids, nucleic acids, nitrate nitrogen, ammonia nitrogen, and adenosine triphosphate. We also analyzed biological and environmental factors (carbon dioxide, illumination, and soil nutrients) that affect the allocation of leaf nitrogen. With respect to biological factors, a higher amount of nitrogen is assigned to Rubisco, resulting in an increase in the photosynthetic nitrogen use efficiency of plants. In most C₃ plants, more than half of the total leaf protein is soluble; in C₄ plants, a quarter of the total leaf protein is soluble. In herbs, a higher amount of nitrogen is allocated to Rubisco. In order to rapidly occupy an area, an invasive plant species assigns less nitrogen to the cell walls and more to the chloroplasts. With respect to environmental factors, illumination affects the distribution of leaf nitrogen, mainly because it affects the energy supply for photosynthesis; this results in a change in the amount of nitrogen assigned for light and dark reactions. Carbon dioxide is required for photosynthesis, and changes in the carbon dioxide concentration in the environment will affect photosynthesis, photosynthetic nitrogen use efficiency, and the distribution of leaf nitrogen. Changes in the nitrogen content in the soil will directly affect the nitrogen content in plants and, in turn, affect leaf nitrogen allocation. Changes in the content of other elements in the soil will also affect leaf nitrogen allocation. We have reported on the methods frequently used for measuring leaf nitrogen allocation. There are two commonly used methods: the formula method and chemical separation method. We have also discussed different perspectives for future research on leaf nitrogen allocation.