<p>Quantum defects with controllable optical and spin properties in solids have emerged as key platforms for quantum sensing, communication, and computation. In this Review, we examine the experimental and theoretical research frontiers on quantum defects. We discuss diverse host materials, including established wide-band-gap semiconductors and two-dimensional materials, and how their structural and electronic properties enable stable and coherent defect states. On the theoretical side, we summarize recent progress in first-principles methods for describing excited states and spin-related properties, and graph-based machine learning methods for predicting defect properties. Finally, we outline key challenges in experimental benchmarking and defect modeling, and point to future opportunities for inverse design and systematic discovery of functional quantum defects for next-generation quantum technologies.</p>