  We describe the pioneering contributions of Thomas K. Gaisser to the birth
and development of particle astrophysics, a new field of research at the
intersection of cosmic ray physics, astronomy, astrophysics, and particle
physics that has emerged in the last few decades. We will especially focus on
his studies of natural beams of neutrinos: those generated by the interactions
of cosmic rays in the Earth's atmosphere and those emitted by astrophysical
sources. Tom actively participated in the discovery of these cosmic neutrinos
as well. His contributions also extend to gamma-ray astronomy, the study of the
cosmic ray spectra and composition, and the modeling of cosmic ray interactions
in the atmosphere and in astrophysical environments. Tom invariably focused his
research on the theoretical and phenomenological problems of greatest interest
at the time, producing frameworks that transparently interpreted often complex
data. These studies have been very influential and have shaped the development
of the field.
