High Energy Cosmic Tau Neutrinos

I discuss the possibility of production of high energy cosmic tau neutrinos ($E\ge {10}^6$ GeV) in an astrophysical site and study some of the effects of neutrino mixing on their subsequent propagation. I also discuss the prospects for observations of these high energy cosmic tau neutrinos through double shower events in new km${}^2$ surface area under water/ice neutrino telescopes.