One of the greatest accomplishments of the theory of general relativity in the past century is the proof of the positive mass/energy theorem for asymptotically flat space-times. However, the concept of mass/energy remains a challenging problem because of the lack of a quasi-local description. Most observable physical models are finitely extended spatial regions and measurement of mass/energy on such a region is essential in many fundamental issues. In my talks, I shall first survey several notions of quasi-local mass including the Hawking mass, the Bartnik mass, the Brown-York mass, and their applications. At the end, I shall describe a new proposal of quasi-local mass/energy and discuss its application in the invariant mass conjecture of general relativity.