Technical Abstract: This chapter describes the neutron thermalization method for soil water content measurement, one of the most commonly used non-destructive methods. Principles, theory, major interferences, and statistics of neutron emission are discussed. Solutions to possible problems are given. Equipment and its use are described. A discussion of measurement volume is included, along with comparisons to other non-destructive methods. A novel device ensuring accurate depth placement of the measurement probe is described. Field calibration typically results in calibration root mean squared errors <0.01 m**3/m**3. Methods for checking calibrations are described. Safety and environmental concerns are addressed; and the chapter ends with a discussion of advantages and limitations of the method. The chapter will be a useful reference for scientists, engineers, students and agricultural consultants needing to know more about the method. The seventeen references sallow for more in-depth reading.