This conference report summarizes the contributions to, and discussions at, the 21st Joint EU-US Transport Task Force workshop, held in Leysin, Switzerland, during 5-8 September 2016. The workshop was organized under 8 topics: progress towards full-F kinetic turbulence simulation; high and low Z impurity transport, control and effects on plasma confinement; 3D effects on core and edge transport (including MHD, external fields and stellarators); predictive experimental design; electron heat transport and multi-scale integration; understanding power decay length in the SOL; role of the SOL in the L-H transition; validation of fundamental turbulence properties against turbulence measurements. This report follows the same structure.