<p>Achromatic metalenses, planar diffractive devices structured with subwavelength nanostructures, are central to the development of compact, integrable alternatives to bulky multi-element optics. Yet their progress is fundamentally constrained by chromatic dispersion, which imposes trade-offs among operational bandwidth, numerical aperture (NA), aperture size, and efficiency. In this review, we introduce a unified framework that classifies achromatization strategies into four distinct paradigms: dispersion engineering, algorithm-aided method, architecture modification, and wavefront engineering. For each category, we analyze the underlying mechanisms, performance trade-offs, and fabrication limits, while identifying opportunities for cross-strategy synergies. Building on this framework, we highlight a forward-looking challenge: the realization of high-efficiency, centimeter-scale broadband achromatic metalenses from visible to infrared, an essential milestone toward application-ready flat optical systems in imaging, sensing, and augmented/virtual reality. This classification and roadmap aim to guide future efforts in overcoming long-standing bottlenecks of dispersion, scalability, and manufacturability.</p>