Much like the valence electrons in an atom, the electrons at the Fermi surface of a metal are the electrons in the highest occupied energy levels. Because of the proximity of these electrons to unoccupied states, all low energy excitations (such as electric and magnetic fields, thermal gradients, mechanical strains, etc...) happen at the Fermi surface. This makes the Fermi surface an object of great interest when trying to understand the properties of a metal, and when trying to come up with microscopic theories to explain phenomena like magnetism and superconductivity. I will provide an introduction to the concept of the Fermi surface, and discuss how we can use the quantization of electron orbits in a magnetic field to measure Fermi surface geometry in momentum space. I will discuss how we used this effect to measure the Fermi surface in YBCO, a high temperature superconductor, and how using special data analysis techniques we also extracted information about the spin susceptibility and effective mass of the conduction electrons.