This book provides a pedagogical introduction to the likely sources of these neutrinos, their propagation and detection mechanisms. Detection of high energy neutrinos of extragalactic origin has led to an interdisciplinary field of research, involving astronomy, astrophysics and particle physics. An extensive review of various detectors and the observations is provided that consolidates the latest findings. Above a few tens of TeVs, neutrinos are conceived as more reliable messengers for astronomy than photons as these photons get absorbed in the background photon field. Determining the neutrino spectrum not only helps in exploring astrophysical objects like AGN, GRB, etc. but also allows us to study particle physics at unprecedented energies. This introductory book is intended to help advanced undergraduate and graduate students to get into the subject with ease, and it simultaneously caters to practicing theoretical or experimental physicists as a reference book.