<p>Evasion of apoptosis is a central hallmark of cancer and a major driver of therapeutic resistance. While genomic and transcriptomic profiling have reshaped our understanding of cancer, they often fail to predict whether a cell will die when treated. BH3 profiling and its variants solve this by directly interrogating mitochondrial apoptotic priming and its reliance on anti-apoptotic dependencies by directly exposing cells to defined BH3-only peptides. Over the past decade, researchers have successfully applied these assays across a wide range of hematologic malignancies, solid tumors, and resistant cancer models. This functional assay predicts treatment response, identifies rational drug combinations, and reveals mechanisms of both intrinsic and acquired resistance to apoptosis. In hematologic malignancies, functional dependence on BCL-2 measured by BH3 profiling strongly correlates with sensitivity to venetoclax, while reduced mitochondrial priming underlies resistance. In solid tumors, these analyses have uncovered previously unappreciated reliance on BCL-xL and MCL-1, enabling the design of effective combination strategies despite limited single-agent activity of BH3 mimetics. Recent methodological advances, including high-throughput and dynamic BH3 profiling, further highlight the changing and heterogeneous nature of apoptotic regulation within tumors. This review evaluates the preclinical and early clinical evidence for BH3 profiling in precision oncology. Beyond assessing its current performance, we examine the methodological requirements for its transition into clinical settings and its potential to address the limitations of current personalized cancer therapies.</p>