Imaging in the atmosphere is often degraded by scattering due to atmospheric particles such as haze，fog and mist.The task of visibility enhancement techniques is to correct for the loss of contrast and color fidelity，which can lead to large improvement in image quality.In this paper，we present a review on established approaches for visibility enhancement of images taken under foggy and hazy conditions.These approaches are grouped into two categories，namely，physics based and non physics based.We illustrate them in detail and then characterize their strengths and limitations，respectively.Additionally，some reference-free objective quality metrics are provided for quantifying the performance related to contrast and chromatic diversity.Finally，further discussions are pointed out on technical challenges and future development