The quest for the unification of fundamental interactions has become the most challenging frontier of sciences in the 21st century. This book presents a detailed analysis and systematic investigation of the foundations of the hyperunified field theory (HUFT) in light of the path integral formulation with the least action principle. Alternative to other unification theories, the starting point of HUFT is initiated from a simple notion that the universe is made of the fundamental building block which is always moving and obeys the basic rule. Such a rule is delved into in this book by proposing the maximum locally entangled-qubits motion principle together with the scaling and gauge invariance principle. These two basic guiding principles are demonstrated to lay the foundations of HUFT, which enable enables us to discuss a series of long-standing fundamental questions, such as: why does the fundamental building block of nature appear as an entangled qubit-spinor field? what brings about the fundamental symmetry of nature? how does the inhomogeneous hyperspin gauge symmetry govern all basic forces? what is the nature of gravity and space-time? how can the space-time dimension and qubit-spinor field be categorized? why do we live in a universe with only four-dimensional space-time? why are there more than one family of leptons and quarks? how does the early universe evolve to be inflationary? what is the nature of dark matter and dark energy?Foundations of the Hyperunified Field Theory will be of great interest to graduate and senior undergraduate students, junior and senior researchers in theoretical physics, quantum field theory, particle physics, gravitational theory, cosmology, as well as mathematical physics and general physics.