The great thing about science, and physics in particular, is that we are always at the brink of discovery. The goal of this class is to understand current developments in theoretical high-energy physics, building on fundamental principles and with a focus on simple but deep ideas. After reviewing the pillars of 20th-century physics, including relativity (special and general), quantum mechanics, and quantum field theory, we will explore the Standard Model of particle physics and survey exciting recent developments such as string theory, extra dimensions, and the AdS/CFT correspondence. Mathematical formalism will be emphasized.