Abstract: Clouds are regular and important weather phenomenon which play important role on the earth's radiation budget and climate change. The shape, size, distribution and movement of them indicate the condition of the atmosphere, thus cloud observ ation is important. Clouds can be observed by means of both satellites and ground based instruments. Satellite cloud images provide global coverage data which are used widely in weather forecast. Ground based cloud images are very localones which contain more details. Ground based cloud sounding instruments have not been put into actual operations by far though. Some cloud base height measurem ents such as ceilometers and the newer laser beam ceilometers are developed in order to determine local cloud base height, however cloud coverage and typology are still determined with a subjective aspect by human observers.In recent years, a number of ground based sky imagers have been developed due to the improvements in both hardware and digital image processing techniques. Many well known instruments are developed and used for cloud coverage estima tion and scientific experiments. For example, the all sky imager (ASI), total sk y imager (TSI) and whole sky camera (WSC) measure visible skylight from the entire sky dome during daytime, while the whole sky imager (WSI) provides hemispheri cal coverage with different detection techniques during day and night. The WSI has approximately 70 nm wide imaging bands centered at 450, 650 nm and 800 nm, ide ntifying clouds from red/blue ratios during the day and from star maps at night (with gaps in operation near sunrise and sunset).The infrared cloud imager (ICI) and the Whole Sky Infrared Cloud Measuring System (WSIRCMS) are ground based passive sensors that measures downwelling atmospheric radiance in the 8—14 μm wave length band. These two instruments can identify clouds and calculate cloud am ount continuously with accordant sensitivity during day and night. Some key techniques are analyzed, such as obtaining whole sky images, camera lens protection, cloud detection, cloud coverage calculation, cloud base height calculation and cloud type classification etc. It shows that, visible light based apparatus can hardly be applied in actual operation because they can't work well during night. The WSI can provide data at night, but the consistency can't turn up trumps because of the algorithmic difference during day and night. Images from cloud infrared radiometers have bad spatial resolution which goes aga inst cloud classification. Sensors using infrared detector array can work with no difference in sensitivity during day and night and have good spatial resolution, but needs improving in stability. Fully considering whole sky cloud sounding theories, techniques and applications, there are still a lot of problems before these instruments to be used in actual operations. Their performance should be more firmness. The whole sky cloud base height should be lucubrated. The instrume ntal cloud form classification principle should be proposed to classify cloud with ground based instruments.