In physics, the inability of General Relativity to explain the very small, and in Quantum Mechanics the inability to explain the very large, has led to a search for a unified Theory of Everything that can hold up across extremes. Urban Quarks is an economic analysis of cities that takes this approach one more akin to physics than modern economics. Just as quarks are the fundamental unit of matter upon which all else is built, we take cities to represent the smallest self-contained unit of society. In this column, we explore the forces and constraints that govern how cities grow, change, and interact with each other.