Desiccation-induced soil cracking is of significant interest in several engineering disciplines, which include geotechnical and geoenvironmental engineering, mining engineering, and agriculture engineering. The hydraulic, mechanical, thermal and other physico-chemical properties of unsaturated soils can be predominantly influenced due to cracks. Reliable information of these properties is required for the rational design and maintenance of earth structures taking account of the influence the soil-atmosphere interactions (e.g., for expansive soil slopes, earth dams, and embankments). In spite of significant research studies published in the literature on the desiccation-induced cracks during the past century, the fundamental mechanism of crack initiation and propagation of soils induced by drying and shrinkage is still elusive. For this reason, the focus of this thesis is directed towards understanding the tensile strength of unsaturated soils which is associated with soil crack initiation criterion (i.e. maximum tensile stress criterion). Tensile strength is the key property of soils for interpreting the initiation of soil cracking from a macroscopic point of view. A semi-empirical model is proposed for predicting the tensile strength of unsaturated cohesionless soils taking into account the effect of both the negative pore-water pressure in saturated pores and the air-water interfacial surface tension in unsaturated pores. The proposed model is calibrated and validated by providing comparisons between the model predictions and the experimental measurements on 10 cohesionless soils (i.e. five sandy soils and five silty soils) published in the literature. The proposed model is simple and requires only the information of Soil-Water Characteristic Curve (SWCC) and Grain Size Distribution curve (GSD), which can be obtained from conventional laboratory tests. To investigate the influence of microstructure, a practical and reliable estimation approach for predicting the evolution of the microstructural void ratio of compacted clayey soils subjected to wetting and drying paths is proposed. The microstructural evolution of 13 examined soils were investigated quantitatively using the mercury intrusion porosimetry (MIP) results. The investigated soils include four high-plasticity clays, eight low-plasticity clays and a glacial till which is a relatively coarse-grained soil with some fines. Based on this study, a novel criterion has been developed for identifying different pore populations of compacted clayey soils. The “as-compacted state line” (ACSL) was proposed to estimate the initial microstructural void ratio based on the compaction water ratio. A constitutive stress is derived to interpret and predict the volumetric deformation of compacted clay aggregates. The linear elastic constitutive model is used for predicting the microstructural void ratio of the examined compacted soils following monotonic wetting and drying paths. The developed approach (i.e. the ACSL and the linear elastic constitutive model) is validated by providing comparisons between the predicted and interpreted microstructural void ratios for all the examined soils. In addition to the matric suction and microstructure, the confining pressure also influences the tensile strength of unsaturated compacted clayey soils. The tensile strength tests on a compacted clayey soil by both the direct method (i.e. triaxial tensile test) and the indirect method (i.e. Brazilian split test) were performed. It is found that the tensile strength increases as the compaction water content decreases for the range investigated in this study, which could be explained by the variation of the inter-aggregated capillary bonding force and the change in microstructure. The increase in the confining pressure has been found to induce the change in failure mode (i.e. from pure tensile failure mode to combined tensile-shear failure mode). In spite of limitations associated with the Brazilian split test, tensile strength is widely determined using this test due to the simple procedure of specimen preparation and wide availability of test equipment in conventional laboratories. However, the Brazilian tensile strength is found to overestimate the tensile strength of compacted specimens with water content greater than the plastic limit. This is due to the considerable plastic deformation associated with the ductile failure instead of brittle failure. In summary, this thesis is devoted to providing insight into the fundamental mechanisms associated with the desiccation-induced crack initiation by quantitatively investigating the various factors that influence the tensile strength of unsaturated soils, which include the matric suction, the microstructure, and the confining pressure from theoretical studies and laboratory investigations.