Abstract: This thesis investigates the phase behavior of two colloidal systems. In the first part, a hard sphere system consisting of PMMA-PHSA spheres(poly-(methyl methacrylate) spheres with a grafted layer of poly-(12-hydroxy steric acid)) in organic solvent is studied using the electric bottle method. A wedge-shaped cell is used in this method. By applying the electric field, a density gradient of the system is obtained and the liquid-crystal transition is found. The experimental results agree with those from a quantitative theory. The wedge cell is also very useful in the studying of glass transitions. In the second part, an anisotropic colloidal system consisting of plate-like(disk) particles is investigated. The plate-like particles are fabricated using photolithography. These particles are birefringent. Their response to an AC electric field is also studied. In a horizontal field they will form ribbon-like chains while in a vertical field they can stand on their edges when the field is high enough. These standing disks behave similar to two dimensional rectangles. The phase diagram of these standing disks is obtained and a Kosterlitz-Thouless(KT) transition from isotropic to nematic phase is found. Near the KT transition, a tetratic-like region is also found, where the tetratic correlations are of a longer range than the nematic. The results from the analysis of topological defects show that this occurs due to the proliferation of Ising-like grain boundaries that disrupt the nematic order but preserve tetratic correlations, at lengths shorter than the spacing between free disclinations.