Abstract: Thorne-Zytkow objects (TZOs) - stars with the outward appearance of red supergiants supported by neutron degenerate cores - are some of the most mysterious and compelling stars in the universe. Thought to be formed from the mergers of binary massive stars, they represent a completely new model for stable stellar interiors, a distinct evolutionary channel for massive binaries and compact objects, and a novel channel for producing the heavy elements that have enriched our universe. While their existence was predicted in the 1970s, no compelling proof of these stars existed until our observation of the first TZO candidate in 2014. In the intervening decade we have continued to study TZOs, a topic that also encompasses a number of open questions surrounding red supergiants and massive binary evolution. This talk will present the latest results of our ongoing work on TZO progenitors and formation, the multi-messenger observational signatures of TZO candidates, and the implications for our understanding of the evolution and final fate of massive stars.