K-theoretic enumerative invariants are defined by holomorphic Euler characteristics of coherent sheaves on moduli spaces. In this talk, I will give an introduction to quantum K-theory whose definition involves moduli spaces of stable maps to given target spaces. I will mention its connections to birational geometry, combinatorics, and number theory. In joint work with Yang Zhou, we proved wall-crossing formulas of quantum K-invariants for any orbifold GIT quotient. These formulas can be used to compute quantum K-invariants. When the target space is an orbifold, quantum K-theory turns out to be quite different from its cohomological counterpart--quantum cohomology. I will present some of these new phenomena.