This document summarizes research on backfilling technologies for underground oil shale mines in Estonia. It discusses how backfilling mine voids with waste rock and ash can reduce environmental impacts by decreasing surface disposal needs. Studies included modeling mine spaces, testing fill materials, and evaluating technological schemes. Laboratory tests showed lower ash mixtures had higher strength. Mine tests indicated warming from large mixtures improved hardening. Overall the research aims to determine if backfilling is technically feasible under local mining conditions.