Within the opening decades of the Twentieth Century, our understanding of the Universe was radically and profoundly changed. During this period, increasingly precise measurements of phenomena occurring at high speeds precipitated a radical revision in our conception of the nature of space and time which is encapsulated in the principles of special relativity. At the same time, experiments which probed shorter and shorter distance scales illuminated the structure of the atom and the properties of even more basic building blocks of matter. Even more importantly, these experiments revealed that at the most fundamental level, these building blocks behave according to the bizarre and often counterintuitive principles of quantum mechanics. In this course, we will focus on these “modern” developments in physics – including special relativity, quantum mechanics, nuclear physics, solid-state physics, and the physics of elementary particles – which transcend the “classical” physics of the previous era. Along the way, we will grapple with quantum uncertainty, discover why Einstein's famous relation E = mc^2 is really so important, and investigate why chemical bonds form. Without exception, none of these developments was the result of a single scientist working in isolation, but rather the product of a community of physicists – in conjunction with chemists, mathematicians, and astronomers – who were influenced by each others' work and who interacted in ways very similar to the way physicists interact today. Out of these interactions emerged a coherent theoretical picture which has been robustly confirmed by experiment. However, it's easy to forget that during the period in which this picture was being developed, things were far from coherent! Indeed, in this course, you will not only learn the physical principles underlying quantum mechanics, relativity, and the other topics we'll be covering (and have a chance to confirm some of them experimentally), but along the way, you'll also get a glimpse of how this body of scientific knowledge was formed – complete with its controversies, false starts, and perplexing experimental surprises.