This text is written for a one-semester Modern Physics course and is designed for students who already have a grounding in basic physics and calculus. Relativity and quantum ideas are cosidered first to provide a framework for understanding the physics of atoms and nuclei. The theory of the atom is then developed, with emphasis on quantum-mechanical notions. This is followed by a discussion of the properties of aggregates of atoms, which includes a look at statistical mechanics. Finally, atomic nuclei and elementary particles are examined.