In recent years, multi-component, high entropy alloys (HEAs) have been proposed as potential alternatives for high temperature structural materials and coatings due to their reportedly favorable combinations of high melting point, high strength, high ductility, and high resistance to oxidation and/or corrosion. HEAs are loosely defined as alloys containing five or more principal elements, each with a concentration between 5-35 at. %. This complex chemical arrangement has been reported to facilitate the formation of solid solution phases consisting of simple FCC and/or BCC crystal structures. Although their potential applications are vast, a fundamental understanding of their high-temperature phase stabilities and oxidation mechanisms, along with effective models to predict their behaviors is deficient. To aid in this gap of knowledge, this dissertation work systematically investigates the phase equilibria and oxidation behaviors of a series of transition metal based HEAs. The phase stability and oxidation studies will encompass both as-melted and annealed HEAs. To critically assess the merit and usefulness of existing thermodynamic databases for predicting complex phase equilibria, the experimental observations will be directly compared with predictions based on the CALPHAD method using ThermoCalcTM. The modeling simulations are applied to both the phase stabilities and the relative oxidation behaviors. The active oxidation mechanisms will also be addressed relative to existing oxide formation models for predicting the oxide growth in alloys with similar elements.