||We present here the All-Sky Transmission Monitor (ASTMON), designed to perform a continuous monitoring of the surface brightness of the complete night sky in several bands. The data acquired are used to derive, in addition, a subsequent map of the multiband atmospheric extinction at any location in the sky and a map of the cloud coverage. The instrument has been manufactured to withstand extreme weather conditions and to remain operative. Designed to be fully robotic, it is ideal to be installed outdoors as a permanent monitoring station. The preliminary results based on two of the currently operative units (at DoÃ±ana National Park, Huelva, and at the Calar Alto Observatory, AlmerÃa, Spain) are presented here. The parameters derived using ASTMON are in good agreement with those previously reported, which illustrates the validity of the design and the accuracy of the manufacturing. The information provided by this instrument will be presented in forthcoming articles, once we have accumulated a statistically significant amount of data.