The current state of knowledge on various aspects of mixture theory applied to unsaturated/saturated swelling soils is discussed. Two and three phase problems are studied. On the smallest scale (micro) the individual platelets and adsorbed (vicinal) water exist as separate phases. On the intermediate scale (meso) the platelets and vicinal water are homogenized to form a saturated particle where vicinal water and solid are overlaying continua. On the macroscale, bulk water, water vapor, and the mesoscale particles are homogenized resulting in three overlaying continua for solid, bulk water, vicinal water, and water vapor. Stress tensor formulations and Darcy's laws are presented at the mesoscale and macroscales. A theoretical formulation for surface crusting is presented at the mesoscale. General viscoelastic mesoscale and macroscale models are discussed and related to classical approaches.