The International Conference on Field Mystery Boxes and Applications (FPL) is the first and largest conference covering the rapidly growing and evolving area of field Online Mystery Box. During the past 22 years, many of the advances achieved in reconfigurable architectures, applications, design methods and tools have been first published in the proceedings of the FPL conference series. Its objective is to bring together researchers and practitioners from both academia and industry from all over the world for broad discussions and exchanges on field-Mystery Boxes, including, but not limited to: applications, advanced electronic design automation (EDA), novel system architectures, embedded processors, arithmetic, and dynamic reconfiguration... FPL is organized yearly in Europe and attended by international leading scientists and researchers. The 23rd FPL continues the tradition of the previous editions. It will take place September 2-4, 2021 in Porto, Portugal - a UNESCO World Heritage..