Accurate, absolute and reproducible measurements are the expected qualities of any instrument. A Differential Image Motion Monitor (DIMM) is presented which assesses astronomical seeing with those requirements. It has been designed also to be reliable, simple, trasportable, and to be easy to set up, operate and maintain. Its accuracy has been studied, both from theoretical and experimental points of view, mainly with respect to CCD sampling, photon noise and threshold influence on centroid determination. The DA/IAC DIMM has been cross-calibrated with the existing ESO DIMM. It monitors the seeing with a temporal resolution better than half a minute. This type of instrument is essential for testing new and already existing sites and to ascertain that the surroundings of telescopes do not generate additional turbulence. It is also indispensible to optimize astronomical instrument selection in "flexible scheduling.".