Detailed structural information about viral envelope proteins is necessary for a better understanding of the viral entry mechanism, as well as for the development of new and improved anti-viral therapies, especially those targeting the pre-fusion and intermediate states of these conformations. The work presented here focuses on using various biochemical and NMR techniques to gather more information on the structure of the envelope proteins of SARS-CoV and HIV-1. Using these techniques I identified an intermediate state of S2-HR2 in the entry of SARS-CoV, which led to a more detailed proposed mechanism along with characterization of the prefusion state of S2-HR2. For the HIV-1 work I was able to use a peptide probe previously described to bind to HIV-1, 12p1, to probe the gp120/gp41 interface and gather details on the conformational changes that occur in this particular region during viral entry