The last two decades have seen a consistent growth in the significance of path integrals. They are useful in quantum as well as statistical mechanics, providing more intuitive and easier methods of handling equations than more commonly-known treatments. However, the method is still greeted with skepticism. Where has the operator ordering problem gone? Do polar coordinates work in PI, despite the fact that they cannot exist in the canonical operator formalism? Is it possible to obtain a quantum mechanics on the sphere by inserting a delta function into the unconstrained formula? This book addresses and solves these fundamental issues, and covers more recent topics, such as the lattice fermion problem in quantum theory and the Monte Carlo method in statistical mechanics.