Abstract: The past centuries have seen great success in photon astronomy, ever refreshing our viewpoints of the Universe. A new type of astronomy has emerged in the past four decades, where the messengers are neutrinos instead of photons. Neutrinos are neutral and nearly massless elementary particles that interact weakly with matter. Thereby they can penetrate dense environments and traverse astronomical distances without being absorbed or deflected. This makes neutrinos a unique messenger to trace the most energetic and distant parts of the Universe. Charged particles called cosmic rays are detected on Earth at the ultra-high energies of 10^20 eV, however the origin of them stays largely unknown even though they were discovered more than a century ago. It is expected that the most violent sources, such as active galactic nuclei, accelerate cosmic rays to the extreme energies, and produce high energy neutrinos with cosmic rays interacting with the ambient materials at the source. Several large neutrino telescopes have been constructed to detect those high energy neutrinos, which will not only provide insight as to the origin of cosmic rays but also shed light on the fundamental properties of matter. The IceCube Neutrino Observatory located at the geographic South Pole is the world’s largest neutrino detector. It has recently detected a diffuse high energy astrophysical neutrino flux consistent with all flavors. I will introduce the IceCube detector, review the detection techniques, prospects of the observed flux, present identification of neutrino flavors and its implications, and discuss campaign searches for high energy neutrino point sources.