Prof. Hisayoshi (“Yoshi”) Yurimoto is receiving the Leonard Medal for his contributions to understanding the microdistribution of oxygen isotopes in the protosolar cloud, protoplanetary disk, and samples returned from asteroids. Prof. Yurimoto became an expert on SIMS (secondary ion mass-spectrometry) analyses during his PhD through using the Cameca ims-3f ion microprobe installed in the 1980s at Tsukuba University, Japan, as a part of “the All-ium project” led by Prof. Naoki Onuma. The instrument was the first SIMS dedicated for geoscience in Japan, and Yurimoto pioneered many analytical protocols to measure trace elements, including hydrogen, and to study element diffusion in solids using SIMS. He was not satisfied just to use the instrument for measurements but pursued his dream to develop new SIMS instruments to solve important scientific questions. Over the past ~30 years, Prof. Yurimoto and his research group at Tokyo Institute of Technology and later at Hokkaido University played a significant role in the development of SIMS as a tool for geo- and cosmochemistry. Some examples include (1) His group designed and constructed the first isotope microscope, consisting of a two-dimensional ion detector called SCAPS (stacked CMOS active pixel sensor) mounted on a Cameca ims-1270/1280, to visualize the distributions of isotopes in a sample. (2) They created a multi-turn time-of-flight secondary ion/sputtered neutral mass spectrometer (TOF-SNMS) with femtosecond laser postionization and nanometer scale spatial resolution. (3) They developed a Cryo-SIMS—a freezing stage installed on a Cameca SIMS. Using isotope microscope, Prof. Yurimoto and his colleagues studied the O- and Mg-isotope distribution in refractory inclusions, chondrules, and matrices in chondritic meteorites. They reported (1) O-isotope heterogeneity within individual anorthite and melilite crystals in CV CAIs, which provide important constraints on the formation of such heterogeneous CAIs; (2) the presence of micron-sized 16O-rich relict grains in chondrules and chondrule igneous rims, indicative of isotopically diverse chondrule precursors; (3) the presence of 16O-rich micron-sized forsterite and enstatite grains in chondrite matrices, suggesting significant radial mixing of solids in the protoplanetary disk; (4) the in situ detection of presolar silicates in primitive chondrite matrices; and (5) the discovery of anomalously 17,18O-rich magnetite-sulfide grains in Acfer 094 (C3.0 ungrouped) named cosmic symplectites (COS). Using TOF-SNMS, high spatial resolution imaging of He isotopes was obtained on Itokawa particle surfaces and Genesis diamond-like carbon. Using Cryo-SIMS, the first ever direct H- and O-isotope measurements of asteroidal liquid water trapped in fluid inclusions in halite crystals from H chondrites were obtained (δD ~+290‰ and Δ17O ~+9‰). This study suggests an exogeneous origin of fluids delivered onto the H-chondrite asteroid from icy objects such as C, P, or D asteroids; comets, or icy satellites of outer solar system planets; and provides important information regarding the origin of Earth's water. Prof. Yurimoto has made many uniquely important contributions to cosmochemistry using SIMS instruments, particularly the distribution of oxygen isotopic abundances in the solar nebula, as well as the chronology and processes related to the origin of the major chondritic components—refractory inclusions, chondrules, and fine-grained matrices. His research on the O-isotope compositions of chondritic components has important implications for understanding the nature of O-isotope reservoirs in the protoplanetary disk, the nature of mass-independent fractionation of O-isotopes in the early solar system, and the mechanism of O-isotope exchange of the chondritic components with nebular gas and asteroidal fluids. His publications on these subjects are a list of firsts. He first described O-isotope heterogeneity in single melilite and anorthite crystals of the Allende CAIs, and attributed it to O-isotope exchange during brief melting event(s). He first described the common presence of uniformly 16O-rich CAIs in CO3.0 carbonaceous chondrites, suggesting formation in an 16O-rich gaseous reservoir. Prof. Yurimoto and his colleagues described an anomalously 16O-rich chondrule in a CH chondrite, and suggested the existence of an 16O-rich reservoir in the solar system that was even more extreme than those sampled by refractory inclusions. In addition to analyzing chondritic components, Prof. Yurimoto modeled a molecular cloud origin for the O-isotope heterogeneity in the solar system and predicted the existence of 17,18O-rich water, as a result of CO self-shielding. This prediction was supported by the discovery of the anomalously 17,18O-rich COS in Acfer 094 and attributed it to the existence of isotopically heavy water, presumably originating in the outer solar system. Prof. Yurimoto has played a leading role in O-isotope measurements of samples returned from the Itokawa asteroid by the JAXA's Hayabusa mission that proved a connection between S-type asteroids and ordinary chondrites. He is currently a Co-I of the JAXA's Hayabusa2 sample-return mission, a collaborator of the NASA's OSIRIS-REx sample-return mission, and the Head of the JAXA Astromaterials Science Research Group. Yurimoto-sensei has educated 68 students, among which 18 have obtained PhD degrees and 25 master’s degrees under his supervision. He has also supported 12 postdocs. Of these students and postdocs, ~15 are actively working in cosmochemistry and several are running SIMS and cosmochemistry labs, including Dr. M. Ito (JAMSTEC, Japan), Dr. T. Kunihiro (Okayama U., Japan), Dr. K. Nagashima (U. Hawai'i, USA), Dr. S. Itoh (Kyoto U., Japan), Dr. T. J. Fagan (Waseda U., Japan), Dr. N. Sakamoto and Dr. N. Kawasaki (Hokkaido U., Japan), and Dr. C. Park (KOPRI, South Korea). Madame President, it is a great pleasure and honor to present to you Hisayoshi Yurimoto for the Leonard Medal.