Quantum tunneling of electrons was used in the 1950's to help develop a precise understanding of mechanism of superconductivity in elemental superconductors. Since the early 1990's researchers have been attempting to use the same techniques to determine the mechanism of superconductors in high temperature cuprate superconductors. In this talk, I will show why this has been such a challenging problem from the perspective of both theory and experiment. The cuprates require far more advanced techniques, especially those based on the scanning tunneling microscope, which allows precise tunneling experiments on the atomic scale. I will review what has been accomplished and describe very recent experiments, which lay a clear path in making progress in understanding these complex superconductors. More specifically, I will address one of the key questions in the cuprates, i.e. at what temperature do pairs in the cuprates actually form and what determines this temperature?

*) Work done in collaboration with K. Gomes, A. Pasupathy, A. Pushp, S. Ono, and Y. Ando.