The effective management of soil water conditions is essential for ensuring healthy plant growth and optimum yield. Drainage is the primary technique by which producers keep soil water conditions at or near optimum for plant growth. A well-designed drainage system may result in a number of benefits including: better soil aeration, less flooding in low areas, less surface runoff, better soil structure, better root development, higher yields and improved crop quality. This chapter begins with a brief summary of the history, purpose and practice of land drainage. This is followed by a review of the literature on the impact of excess water and drainage on different aspects of soil health, including physical, chemical, and biological properties and processes.