It has been over two decades since M. Gromov initiated the study of pseudo-holomorphic curves in symplectic manifolds. In the past decade we have witnessed mathematical constructions of Gromov-Witten theory for algebraic varieties, as well as many major advances in understanding their properties. Recent works in string theory have motivated us to extend our interests to Gromov-Witten theory for Deligne-Mumford stacks. Such a theory has been constructed, but many of its properties remain to be understood. In this talk I will explain the main ingredients of Gromov-Witten theory of Deligne-Mumford stacks, and I will discuss some recent progress regarding main questions in Gromov-Witten theory of Deligne-Mumford stacks.