This book is a comprehensive and updated review of fundamental studies on inhibition of soil urease activity and of applied studies on improving efficiency of urea fertilizers by inhibition of soil urease activity. The general literature on these topics covers 65 years and the patent literature comprises a period of nearly 40 years. The potential of food production to meet the growing needs related to population increase is largely conditioned by the efficiency of agricultural fertilizers. Urea has gradually become the most important nitrogen fertilizer in world agriculture. However, its efficiency is in general reduced due to excessive activity of a soil enzyme, urease. One way to increase efficiency of urea fertilizers is the inhibition of soil urease activity. In the last four decades, multilateral investigations have been carried out in a series of countries to identify and test unpolluting and inexpensive chemical compounds to be used as inhibitors of soil urease activity. These investigations are reviewed, including those described in the patent literature. The book is addressed to a broad audience, including experts and students in agronomy, forestry, plant physiology, soil science (especially soil biology and biochemistry), and other environmental sciences, as well as in organic and inorganic chemistry.