This lecture covers the fundamental concepts of High Entropy Alloys (HEAs), including their composition, formation, and thermodynamic considerations. It discusses the different classes of HEAs, stability criteria, and the influence of electron concentration and crystal structure on their properties. The lecture also explores the mechanical properties, fabrication methods, and unique characteristics of Bulk Metallic Glasses (BMGs). Current research trends, such as refractory HEAs and high-strength low-density HEAs, are highlighted, along with the challenges and potential applications of these innovative materials.