This chapter reviews how humic substances (HS) can be used as plant biostimulants in agriculture. It begins by describing how humic substances are produced and highlights different characteristics of these substances. It then goes on to review how humic substances can be used in terms of plant growth and their effects on the physical and chemical properties of soil. The chapter also provides a section on the biological activities of humic substances, and how humic products are used commercially. The chapter concludes by corroborating all the research together and emphasises the main issues encountered when studying the effects of humic substances on plants and soils.