Recent high-resolution observations from the IRIS satellites have enabled significant advancements in our understanding of the physical mechanisms at play during the impulsive phase of flares, including details of how the non-thermal energy is released and propagated from the corona to the low-atmosphere through accelerated particles. At the same time, the new discoveries have brought to light new unsolved questions and challenges for current models. This talk will provide some examples of the unique contributions to our understanding of flares from IRIS, also in coordination with other solar observatories, and how state-of-the-art heating models of flares can be constrained by the available imaging and spectral diagnostics. I will also discuss some of the outstanding problems in preparation for the next generation of solar missions.