Understanding the molecular-level chemistry underlying cellulose decomposition can help chart a course to more efficient processes to convert this abundant biopolymer into renewable chemicals and fuels. This fundamental knowledge had remained elusive, despite decades of research into the chemical mechanisms governing its conversion. To solve these mysteries, we have created multiscale models of thermochemical and enzymatic conversions of cellulose that allow us to investigate complex reaction networks, determine elementary steps, calculate kinetic parameters and compare model results with experimental data. This work highlights how advanced computational tools can uncover notoriously complex carbohydrate chemistry that we can use to advance renewable chemicals and fuels.