This invited paper considers reasons why exact measurements of fast electron and ion losses in tokamaks. and particularly i n a scrape-off-layer and near a divertor region. are necessary in order to master nuclear fusion energy production. Attention is also paid to direct measurements of escaping fusion products from D-D and D-T reactions. https://www.ngetikin.com/quick-offer-1-Gallon-Universal-Primer-Pro-Deep-Penetrating-Primer-big-deal/