If we examine the EIA process in different countries, it seems that it is linked to a previous phase of discussion of projects, through the administrative process.This previous stage is, in fact, one in which the project can be publicly discussed and the decisions about location, design, etc., are made, and one in which the preventive focus of the EIA can be applied. In consequence, the EIA process looks as if it were linked to the pre-construction period, although, in fact, it must be a cyclic process, with some control instruments to guarantee the performance of mitigation measures and auditing effect forecasts. However, of central interest in determining the effectiveness of the EIA is the extent to which the environment is managed and protected as a result of the whole EIA process. Environmental auditing is an important tool for providing an account of construction and post-development (EIA) activities. In this paper, we analyze the experience in auditing programs of project and construction of highway projects in Spain, in the last ten years, concluding the importance of auditing programs to guarantee the success of the EIA process, and establishing the basis of its contents, structure and implementation, particulary in the field of ecological impacts. The paper discusses the content and structure of auditing programs, the agents involved in the process, and the responsibilities of each one looking at the experience in Spain. This analysis permits, also, identify the frequent practice in preventive, corrective and compensatory measures in the highway projects in Spain. This paper reflects, in part, the conclusions derived of a research project financed, during 2002, by the Centro de Estudios y Experimentación de Obras Públicas (CEDEX, Research Centre of Public Works) of the Spanish Ministry of Public Works (Ministerio de Fomento), througth the analysis of more than 40 highway projects developed in the last ten years in Spain, most of them already constructed. Moreover, it reflects, also, the results of the works related with the Quality Verification of Environmental Annexes made by the authors for ESTEYCO, technical assistance during several years of the Spanish Ministry of Public Works for the Quality of Projects Assurance Plan.