In Chap. 6, we saw that drastic changes in various physics around string theory have been brought from string theory, by the discovery of D-branes. Although each of these are very meaningful and interesting developments, on the other hand how about developments in string theory itself? How has string theory been developed by advent of D-branes? String theory is a candidate theory unifying all the elementary particles and their interactions. What extent has string theory been understood as the ultimate theory to?