Today, due to a positive economic outlook, Ethiopia is planning and implementing ambitious investment projects. Dam construction projects are one of investment projects play a major role in this regard. However, dam construction projects are facing challenges to provide a successful project delivery due to starting projects with ambiguous and uncertain projects scope requirements. This study aims to investigate the extent of scope definition practices by using PDRI for infrastructure projects, the challenges for scope definition and impacts of poor scope definition practices on dam construction projects in Tigray Region. The data for the extent of scope definition practice was gathered through retrospective case study approach by rating the level of scope definition of PDRI elements referring to feasibility study reports by experts. The result indicates dam construction projects scope definition practice is not adequately performed, as far as the PDRI score for all dam construction projects were 287 to 492, which is above the recommended scope definition practice of PDRI score which is below 200. Furthermore, the data collected through retrospective case study the ten most poorly defined elements will acts as potential risk factors were identified and analyzed through percentage of the PDRI score of each elements, projects funding and programming, geotechnical and geological characterization, project delivery and contracting method, safety and hazards, right of way and site issues, value engineering procedure, hydrological characteristics, capacity study, contingencies’ and material alternatives considered and project cost control are the ten most poorly defined scope elements on the dam construction projects. Furthermore, the study also revealed that, a list of challenges for scope definition practices were investigated through data collected from, questionnaire survey and supplemented by interview and case study and analyzed by descriptive mean score statistics were; insufficient resource (equipment limitation, time and fund) for scope definition, inadequate involvement of all relevant stakeholders and insufficient trained and competent professionals, not following formal decision approval procedure and lack of coordinated document management system and political interferences were among the others. Eventually, the study also revealed that, a list of impacts of poor scope definition practices were investigated through data collected from, questionnaire survey and supplemented by case study and physical observation and analyzed by descriptive mean score statistics were; cost overrun, time overrun, change order, re-works, project failure, a low morale and productivity work force were among the other impacts. Based on the results of the study recommendations were forwarded to improve effective scope definition practice on dam construction projects.