This lecture, the first in a series on neutrinos, explores the fundamental role of these elusive particles in physics, astrophysics, and cosmology. Delivered by Mariam Tortola at the Galileo Galilei Institute (GGI), the presentation delves into the theoretical foundations and experimental evidence surrounding neutrinos, examining their unique properties and significance across multiple scientific disciplines. Learn about neutrino oscillations, mass hierarchies, detection methods, and how these ghostly particles inform our understanding of stellar processes, cosmic evolution, and the fundamental laws of physics.