Topography and ground conditions for mountain road engineering within the Central Cordillera of Luzon in the northern Philippines (Fig. 1) are some of the most extreme and unstable in the world. The Central Cordillera is a range of mountains that has been emplaced and shaped by rapid uplift, valley incision, slope erosion, and often massive landslides (Fig. 2). The Halsema Highway is a 180 km-long,tortuous road that traverses this mountain range and is a vital economic and strategic lifeline to the intensively farmed agricultural mountainous regions of northernLuzon. In 1990 the road suffered extensive loss and damage as a result of a Richter magnitude 7.8 earthquake followed by a succession of typhoons and intense rainstorms. Given the strategic and socio-economicimportance of the highway, the Philippines Government sought international aid to rehabilitate this vital link following the devastation, and feasibility and design studies commenced in late 1996. This paper presents someof the geotechnical investigation carried out for these studies, describing the types of instability and ground conditions affecting the road and the approach adopted to maximize the interpretation of the ground, andto provide geotechnical engineering options for road rehabilitation.