Metal halide perovskite solar cells (PSCs) have undergone remarkable progress in power conversion efficiency since the first report of solid-state PSC in 2012, yet further advances toward practical photovoltaic technologies require simultaneous control of crystallization, compositional homogeneity, interfacial carrier extraction, and operational stability. In this talk, I will discuss our recent efforts to understand and regulate perovskite materials and interfaces from precursor chemistry to complete devices. At the absorber level, we identified several intrinsic origins of performance loss and instability. Out-of-plane segregation of FA and Cs cations can develop during crystallization, whereas different crystal facets exhibit markedly different susceptibility to moisture-induced degradation. Controlling crystallization and preferentially exposing stable facets therefore provides an effective route toward compositionally and structurally robust perovskite films. More recently, an entropy-regulating molecular lock was developed to stabilize the photoactive α-FAPbI₃ lattice by regulating organic-cation dynamics and suppressing disorder of the Pb-I octahedral framework, enabling a certified efficiency approaching 28%. Equally important is molecular control of interfaces. Homogenization of self-assembled monolayers at the buried interface improves wettability, suppresses nanovoids and interfacial stress, and reduces non-radiative recombination. At the perovskite top surface, regulation of surface crystallization through a solvated intermediate enables direct transformation into the photoactive α-phase, reducing contact losses and yielding efficient and stable single-junction cells, and all-perovskite tandems. We further reveal that molecular interfaces are chemically active during film formation. Phosphonic-acid self-assembled monolayer can trigger DMSO-mediated iodide oxidation, while chemistry-matched additives redirect these reactions, enabling 27.7% laboratory cells and 20.1% efficiency in 2-m² modules. Finally, reconfiguration of lead-iodide octahedral connectivity provides a new means of creating complementary electron- and hole-selective interfaces without conventional extrinsic doping, producing efficiencies exceeding 27%. Together, these studies demonstrate that coordinated control of precursor chemistry, crystallization, lattice stability, and interfacial molecular structure provides a pathway toward efficient, stable, and scalable perovskite photovoltaics.