Ultra-high dose rate FLASH radiation has had a recent resurgence in the radiation biology, chemistry, physics, and oncology communities. However, examination of the literature shows a significant number of earlier key studies from 1900s to 2000s that demonstrated that FLASH delivery of radiation can spare normal tissues and cell killing. Mechanisms were proposed including oxygen depletion, radical : radical recombination, and issues with dosimetry. While recent FLASH investigators have acknowledged this prior work, it is important for those new to the field to understand and appreciate these foundational earlier studies to better guide new research in this exciting area. In this talk, key earlier published ultra-high dose rate studies and proposed mechanisms will be presented and discussed.