Technological advances driven by modern experiments provide a context to examine fundamental questions in quantum physics. For example, the ability to control and probe single quantum systems led to tests of fundamental effects such as superposition, entanglement, and decoherence, which enhanced the development of quantum information. This control is currently being extended to systematically build many-body systems from individual components and engineer the environment that surrounds them. Arrays formed from single atoms kept in place by light, as well as superconducting circuits coupled to a common environment are now being constructed in laboratories around the world. In this talk, I will describe the collective optical response of these arrays and explore fundamental differences that emerge with increasing system size. I will discuss how these arrays can be used as toy models for complex materials, probe quantum light, and to develop efficient light matter interfaces. I will finish by presenting new theoretical tools that need to be developed to harness modern quantum technologies and questions that arise within this framework.