Soil organic matter (SOM) is the largest reservoir of organic C in the biosphere, however its composition and structure remain uncertain. The role of humic substances as the main component of SOM is currently disputed. Here, we are discussing the problems of extraction and fractionation of the components of SOM, the aspects of investigation of its structure by spectroscopic methods, the problem of humification in relation to the origin of SOM. We are also discussing the role of organic matter of permafrost soils and peats in relation to carbon cycle under global climate change. The direction of further investigations and future tasks are outlined.