This study examines the background of the PDL(Personal Digital Library) as well as a conceptual understanding of the PDL in knowledge-based society. Recent case studies are also included. The chief concepts of the PDL are Systems(Digital Library, Knowledge Management System, Internet Portal Site), Models of knowledge, and Personalization. The findings in this study are as follows. ① We must find the meaning in the change of a society environment in which the personalization happens. ② The contents must not be restricted. ③ The system is to satisfy requirements of the individual. ④ The GNU General Public License is a system development method for the PDL.