Most existing social network analysis has focused on networks with only positive links (or unsigned networks). However, in many real-world social systems, relations between two nodes can be represented as networks with both positive and negative links, where negative links can denote foes, distrust, “friended" and “followed" friends, and blocked users. The introduction of negative links in signed networks not only increases the complexity, but also poses tremendous challenges for traditional unsigned network analysis. It is evident that negative links have distinct properties from positive links; and the fundamental principles and theories of signed networks are substantially different from those of unsigned networks. Hence, dedicated efforts are desired for signed networks. In this talk, I will systematically introduce our work on signed social network analysis from foundations to applications, which suggest great opportunities in this new research subfield.