The soil moisture is one of the factors that affect the crop production. The plants require an adequate amount of soil moisture that may vary according to the crop species and stage of growth or development of a plant [1]. The soil can only store a limited amount of water, and only a part of this storage is available to the plant. For this reason, it is essential to know the soil moisture content per unit mass or per unit soil volume, and its water potential or availability of the soil moisture. This provides

valuable information to understand many of the chemical, mechanical, and hydraulic properties of the soil. This information helps to design an efficient irrigation system for supplying water to the soil for the plant use. Different methods have been developed to determine the soil moisture. The use of each of these methods depends mainly on the economical resources of the operator, his knowledge and a desirable degree of precision. This chapter discusses basic principles of soil, water, and plant relations; and the use, operation, advantages, and disadvantages of various methods to determine soil moisture. We hope that this information can enrich the knowledge of the farmers, scientists, and agricultural technicians.