LIDAR (Light Detection and Ranging) is one of the most recent technologies in surveying and mapping. LIDAR is based on the combination of three different data collection tools: a laser scanner mounted on an aircraft, a Global Positioning System (GPS) used in phase differential kinematic modality to provide the sensor position and an Inertial Navigation System (INS) to provide the orientation. The laser sends towards the ground an infrared signal, which is reflected back to the sensor. The time employed by the signal, given the aircraft position and attitude, allows computation of the earth point elevation. In standard conditions, taking into account the flight (speed 200 -250 km / hour, altitude 500 -2,000 m) and sensor characteristics (scan angle ± 10-20 degrees, emission rate 2,000-50,000 pulses per second), earth elevations are collected within a density of one point every 0.5-3 m. The technology allows us therefore to obtain very accurate (5-20 cm) and high resolution Digital Surface Models (DSM). For many applications, the Digital Terrain Model (DTM) is needed: we have to automatically detect and discard from the previous DSM all the features (buildings, trees, etc.) present on the terrain. This paper describes a procedure that has been implemented within GRASS to construct DTMs from LIDAR source data.(DSM), which describes the Earth's surface, including all the objects on the ground and the digital terrain model (DTM) reproducing the "natural" ground surface, i.e. the bare Earth surface. Both surfaces are being widely used in many fields such as topographic mapping, urban planning, ecological and environmental studies, flood prevention and drainage mapping, forestry, landscape design and infrastructure construction, maintenance and management. Moreover, with the development of 3-D GIS, the DEMs assume even more importance, because of the central role played by topographic data within GIS.The conventional techniques used to collect altimetric data include land surveying and aerial photogrammetry. In the past few years, a new method to measure the topography of the Earth's surface has attracted interest because of its high accuracy, low time requirements, and competitive or lower costs than earlier methodologies (Shrestha et al. 2000). This method, known as LIDAR (Light Detection and Ranging) or ALSM (Airborne Laser Swath Mapping) has a very simple measurement principle (Baltsavias 1999, Wehr and Lohr 1999, NOAA-USGS 2002. It is given by the combination of three different types of equipment: a laser scanner, the global navigation system (GPS) and an inertial navigation system (INS). The laser scanner is installed on the aircraft as the phogrammetric camera usually is. Laser pulses are emitted towards ground with high repetition rates per second (2,000 up to 50,000) and are reflected back to the instrument. A mirror inside the laser transmitter rotates in a sweep motion perpendicular to the direction of flight in order to blanket the surface of the Earth in a strip buffering this direction (swath width of up t...