The rate of discovery in quantum correlated materials in recent years has been remarkable. From 2D materials, to topological insulators, to strong spin-orbit coupled systems, to quantized responses in spin-liquids and fractionalization, these advances challenge our previous notions of the possible behavior of electrons in solids. However, it is the case that many of their most interesting properties remain hidden to us. They have distinct quantum mechanical correlations that are in many cases only indirectly accessible with current techniques. For instance, they can have Berry phase structures in momentum space, fractionalized quasiparticles, or many-body entanglement that we simply do not have the tools to characterize properly. There can also be new forms of classical order that are largely invisible to conventional scattering techniques. Going forward it is going to be essential to develop new techniques and instrumentation that can reveal their properties. One of the most promising directions to get qualitatively new information is the use of nonlinear optical spectroscopies and particularly those in the THz range. For a number of reasons nonlinear spectroscopic responses can give unique information about quantum correlations that are completely inaccessible at the level of linear response. In this talk I will discuss my group's efforts to develop and use nonlinear THz spectroscopic techniques to probe unique aspects of quantum materials. I will center on the new technique of THz 2D coherent spectroscopy from both a theoretical and experimental perspective. I will give examples of this technique's use to probe phenomena as diverse as fractionalization in spin liquids, “marginal quasiparticles” in electronic glasses on the insulating side of the 3D metal-insulator transition, and the strange metal state of cuprate superconductors.