Rooted in the author's thirty years of classroom experience, this text strips quantum theory down to its most basic content by deriving its fundamental equations from the fewest possible physical principles. Development of Quantum Theory from Physical Principles presents the entire content of quantum theory and its formalism based on the Uncertainty Principle and the Invariance Principle. The treatment demonstrates that it is possible to establish the existence of specific pairs of conjugate observables and determine their properties. In addition, it relies on the Dirac equation and provides an explanation of the spin-statistics theorem. For undergraduate and graduate educators and students in quantum theory courses as well as physicists working on quantum phenomena.