Despite rapid progress in the computational design of novel functional materials, the materials discovery pipeline remains bottlenecked by the difficulty of synthesizing predicted compounds in the laboratory. Developing a theoretical foundation for predictive materials synthesis requires a more quantitative understanding of metastable phases, which often appear as kinetic byproducts during materials formation. By mapping the thermodynamic scale of crystalline metastability, and calculating relative nucleation rates between competing polymorphs, we can construct synthesis maps to navigate through the thermodynamic and kinetic energy landscape towards desired material phases. I will showcase several applications of this ab initio framework to predict non-equilibrium crystallization pathways of carbonate minerals and functional manganese oxides in hydrothermal synthesis, and conclude with thermodynamic strategies for the discovery and synthesis of novel thin-film nitride semiconductors. Mastering metastability will deepen our fundamental understanding of nucleation and crystal growth, and can expand the search space for functional technological materials beyond equilibrium phases and compositions.