This synthesis will be of interest to state department of transportation (DOT) construction, geotechnical, materials, and pavement system design engineers, engineering geologists, and research engineers, and others concerned with the constructibility of new pavements over existing subgrades. The synthesis describes current practice for the stabilization of existing subgrades to improve constructibility during interstate pavement reconstruction. It presents information regarding the methods available to evaluate and improve subgrade conditions for the purpose of meeting the constructibility requirements of a reconstruction project. This report of the Transportation Research Board presents data obtained from a review of the literature and a survey of the state DOTs. The synthesis reports on: subgrade evaluation methods including sampling, laboratory, and in-situ test methods, as well as assessment of existing drainage systems; constructibility factors such as existing and proposed pavement types, available equipment, and cost effectiveness of various subgrade stabilization techniques; methods of subgrade improvement including mechanical and chemical stabilization, use of recycled and waste materials, the use of geosynthetics in reinforcement and drainage applications; and construction methods with an emphasis on innovative approaches such as novel sequencing of construction traffic, use of lightweight equipment, and robotics. In addition, several case histories describing applicable pavement reconstruction projects are presented. Finally, suggestions to possibly improve the practice and the identification of research needs are also presented.  The report for this topic can be purchased at https://www.trb.org/news/blurb_detail.asp?id=3310