Metalloporphyrins play a starring role in the chemistry and physics of biology, photo-electricity, sensing, catalysis, electrochemistry, and other important areas. Because of this, they have been widely studied (both experimentally and theoretically) in the solid state and in solution. Very recently it has become possible to experimentally probe their chemistry at the single molecule level. Of even more relavence to applications, it is now possible to do single molecule studies at the solution-solid interface. Scanning tunneling microscopy (STM) provides images, at the single molecule level, of reactions between surface supported metal porphyrins and molecular species like oxygen, imidazole, and other reactive species. These scanning probe studies clearly demonstrated that the chemistry of the supported porphyrin can be very different than in solution. Density functional theory has allowed us to more fully understand the nature of the substrate-porphyrin-ligand interaction relative to the substrate free case. We find that whenever the porphyrin is supported on a conducting substrate, there is a complex charge exchange process that can significantly modify the axial binding relative to what is observed in solution. In some cases, the interaction is such that even the spin state of the surface supported metal ion differs from that of the same chemical system in solution or gas phase. We will present examples of experimental and computational studies of this new chemistry.