The third edition provides practicing physicists with the fundamental principles that underlie the behavior of matter while presenting a modern integration of 20th Century physics. Emphasis is placed on constructing and using physical models. Serious computer modeling is introduced in the beginning to help build a strong foundation on the use of this important tool. End-of-chapter problems delve into experiments using simple equipment to gain insight into deep scientific issues. Stop and Think questions are also included to engage physicists in the material.