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Recent progress in metasurface designs fueled by advanced-fabrication techniques has led to the realization of ultrathin, lightweight, and flat lenses (metalenses) with unprecedented functionalities. Owing to straightforward fabrication, generally requiring a single-step lithography, and the possibility of vertical integration, these planar lenses can potentially replace or complement their conventional refractive and diffractive counterparts, leading to further miniaturization of high-performance optical devices and systems. Here we provide a brief overview of the evolution of metalenses, with an emphasis on the visible and near-infrared spectrum, and summarize their important features: diffraction-limited focusing, high-quality imaging, and multifunctionalities. We discuss impending challenges, including aberration correction, and also examine current issues and solutions. We conclude by providing an outlook of this technology platform and identifying promising directions for future research.