Ultra-high dose rate irradiation or “FLASH RT” has recently gained attention because of the surprising observation of markedly increased therapeutic index compared to conventional dose rate irradiation in preclinical studies of in vivo animal models. Most accelerator-based radiation delivery systems used for FLASH were uniquely modified to produce high radiation outputs. However, FLASH RT requires accurate dosimetry in such intense treatment beams. Dosimetry in high dose rate and high dose-per-pulse beams is challenging due to the lack of appropriate radiotherapy dosimetry protocols. Current protocols are not designed for such beam parameters mainly because of the lack of adequate radiation detectors, such as ionization chambers, diodes, diamond detectors, etc. Most of these detectors exhibit non-negligible ion recombination when the dose rate and/or the dose per pulse is increased beyond what is used in conventional radiotherapy. Furthermore, all experiments to date produce pulsed radiation, and there are many aspects of delivery speed, for example, total dose and delivery time, dose per pulse, pulse timing structure, etc., that may be critical to the effect and have not been comprehensively evaluated, since an exhaustive study is an experimentally daunting combinatorial problem. In this session we will discuss the aspects of FLASH dosimetry and potential optimal beam parameters to achieve the maximum FLASH effect.