Welcome to the 21th International Summer Institute on Phenomenology of Elementary Particles and Cosmology (SI2015). The Summer Institutes have been held every summer since 1995. Most previous meetings were held in Mt. Fuji of Japan, except for SI 2006 at Pohang (Korea), the SI 2008 and SI 2012 at Chi-Tou (Taiwan), and SI 2013 at Jiri-Mountain (Korea). This year, the SI2015 will be held in China on Aug. 1 - 7, 2015, at the beautiful Yan-Xi Lake Campus of the University of the Chinese Academy of Sciences，at the northeastern suburb of Beijing (中国科学院大学雁栖湖校区). It is besides the famous scenery park, Yan-Xi Lake, where the recent APEC conference was held. SI aims at an Aspen-type Institute in Asia. The purpose of the Summer Institute is to promote communications and discussions on research among participants, rather than to give pedagogical lectures for students. Therefore the scientific program is relaxed to keep enough time for discussions. We hope that the new ideas and research activities will be initiated from this institute.