Nonlinear optical materials show great promise in a broad range of applications from cancer therapies and medical imaging to increasing the speed of the internet. Making such applications possible requires molecules that interact more strongly with light. After more than three decades of research, many new molecules have been synthesized with larger nonlinear response, made larger mainly by increasing the number of electrons and decreasing the energy gap; but, the best molecules have remained a factor of 30 below the fundamental limit. I will discus how the fundamental limits and scale invariance are applied to the design of better molecules. While there are many ways to optimize the nonlinear response of a quantum system, they all share certain universal properties.