This lecture explores the classes of High Entropy Alloys (HEAs), including those based on 3d transition metals, refractory metals, rare earth elements, and light metals. It delves into the stability criteria for HEAs, the main high entropy effects, and the influence of electron concentration and crystal structure on their properties. The lecture also covers the mechanical properties of HEAs, the current research trends focusing on refractory and two-phase HEAs, and the fabrication and properties of Bulk Metallic Glasses (BMGs).