Since the discovery of D-branes as solitons of string theory by Polchinski in 1995, the application research has been explosively developed. This word “explosively” is not an exaggeration. D-branes have given tremendous influences not only to the framework of string theory but also to various physics around, by their flexibility. They have brought us not a slight influence but a huge revolution such as a creation of new subjects and supply of new paradigms. In this chapter, I will pick up four topics which have been considerably developed intrinsically owing to the emergence of the D-branes, and will have a brief explanation for each. As you can read these four sections independently, you might start with whichever section you are interested in. Since they are leading-edge research results, I am forced to omit details a little bit. However, I hope you may read the great influences given by D-branes, the possibility of their future, and the excitement of researchers engaged in the study of D-branes.