Аннотация:The article discusses the methods of field instrumental measurements and the use of remote sensing materials for measuring the leaf area index of vegetation cover, their capabilities and limitations, verified during research in northern forests. The leaf area index LAI is the ratio of the area of leaves (one of their sides) and/or needles of all plants to the soil area occupied by a given ecosystem. LAI is an important parameter that reflects material and energy metabolism in the processes of photosynthesis, respiration, the cycle of carbon and plant nutrients in ecosystems, predicting their growth and productivity. LAI is a key variable functionally related to phytomass production, water cycle and nutrient cycle under specific microclimatic conditions. A reliable estimate of LAI is one of great importance for monitoring and analyzing various biophysical processesin ecosystems; it is a complex indicator that quantitatively reflects the closeness of the tree canopy, the projective cover of shrub, grass, moss-lichen layers of natural ecosystems or crops of agrocenoses. The LAI is used in environmental studies aimed at studying the state of the vegetation cover for solving scientific and applied problems. The global LAI is 4.5, its values depend both on natural conditions, mainly hydrothermal, and on the characteristics and degree of anthropogenic impact. The theoretical foundations of studying LAI and various methods of its measurement are considered. In field studies, LAI is determined either with the help of special devices or in a destructive way: the assimilation area of leaves is determined by the method of “incisions” of a fixed area, and the accumulation of dry matter – by the gravimetric method, followed by dryingof the vegetative mass of plants to an air-dry or absolutely dry state... The use of remote sensing data is very promising to determine LAI of various types of vegetation without removing phytomass and to assess their condition.