Magnetic random-access memory (MRAM) based on spin-orbit torque (SOT) is a promising non-volatile memory technology for the post-Moore era, owing to its fast switching speed, superior endurance, and potential for low-power operation. However, achieving deterministic current-induced magnetization switching in high-density perpendicular magnetic anisotropy systems, without reliance on external magnetic fields, remains a critical bottleneck, impeding its widespread commercial application. This review surveys recent progress of SOT-driven field-free switching of perpendicular magnetization and gives a coherent overview of symmetry-breaking mechanisms and device-level implications. Strategies that create intrinsic effective fields through engineered structural asymmetry (e.g., wedged layers and asymmetric interfaces) and built-in gradients such as composition or oxidation profiles are summarized. Approaches based on magnetic interactions, including antiferromagnetic exchange bias and interlayer coupling in multilayer and synthetic antiferromagnetic structures, are also discussed. Then, emerging mechanisms implemented by low-symmetry crystals and topological materials are highlighted, in which nontraditional spin textures and out-of-plane spin polarization contribute to deterministic PMA switching in the absence of external fields. In addition, recent demonstrations of SOT-driven self-switching in magnetic single-layer systems are introduced. Finally, opportunities and remaining challenges for SOT-based spintronic devices are outlined in the context of future information technology, with a focus on determined switching, write-current reduction, thermal stability, device variability, endurance, and CMOS-compatible integration.