G2G2-Summer School is associated with G2G2-Minisymposia of 8ECM. It belongs to the G2-series that are about strong and beautiful mathematics, especially those involving group actions on combinatorial objects. The main goal of G2G2-Summer School is to bring together experts and students to exchange ideas and to enrich their mathematical horizon. We organize two short courses and four colloquium talks to let participants see order and simplicity from possibly new perspectives and share insights with experts.