This is an advanced textbook for masters and graduate students studying relativistic quantum theory and, in particular, quantization and renormalization of gauge fields. There are very few existing books on this subject, and no recent books at all. Readers will be brought from basics to the frontier of the field. The knowledge shared in the book will enable them to perform calculations involving higher-spin fields and develop new theories. Topics covered in the book will include Relativistic invariance, Lorentz group, (anti-) de Sitter space, Relativistic fields, gauge symmetries and algebras, Fermionic fields, Rarita-Schwinger construct, supersymmetries, Classical fieldequations, constraints, degrees of freedom counting, Causality, Velo-Zwanziger problem, Path-integral quantization of theories with constraints, Faddeev-Popov ghosts, Perturbation theory, Feynman rules, Renormalization, tree-level unitarity, Micro-causality, dispersion relations, sum rules, Connection with string theory, Vasiliev theories, higher-spin fields in AdS space. All chapters will include pedagogical examples, which can be used as exercises in the course. Interactive notebooks in Wolfram Mathematica will be included, as engaging illustrations of the pedagogical example calculations considered in the text. There will also be FORM (computer algebra) codes for some of the calculations.