A key property of all living organisms is the ability to sense environmental signals and respond to them by modifying behavior. This feature extends from the most complex organisms (e.g., humans) to the smallest unit of life, the cell. It forms the basis of a wide range of biological phenomena that include embryonic development, immune function, and cognition. In this talk, I will focus on one of the simplest examples of "cellular decision making" found in nature: how swimming bacteria navigate their environment, a phenomenon known as chemotaxis. I will explain how unicellular microorganisms like E. coli propel themselves in water, how they sense their surroundings, and how they change their swimming behavior in response to changing environmental conditions. I will describe how researchers are now able to study such processes in unprecedented detail using tools at the interface between the biological and physical sciences. Finally, I will discuss what new insights have emerged and how they relate to decision-making in more complex organisms.