The water supply and sewerage construction projects in Addis Ababa city recurrently facedrnseveral problems which lead to schedule and cost overruns. The objective of this study was tornassess the determinant Factors affecting schedule and cost overruns on water supply andrnSewerage construction projects in Addis Ababa City. This research used a descriptive surveyrndesign and a combination of both quantitative and qualitative data collections methods torncollect primary data as well as secondary data. The research data were collected fromrnprimary and secondary sources using structured questionnaires administered to SiternEngineers, project managers, consultants, contractors, procurement officers, budget/financernofficers, project planning, monitoring and controlling officers, and Internal Auditors. The datarncollected through a structured questionnaire were analyzed with SPSS version 20.0 softwarernand the Relative Importance Index (RII). Documentary evidence was also reviewed by thernresearcher for the extent of cost and schedule overruns case analysis. The study has finallyrnpointed out the most important determinant factors that affect schedule delays on water supplyrnand sewerage construction projects in the city as perceived by the three parties (client,rnconsultants, and contractors) are bureaucracy in Government agencies, Government tenderingrnsystem of choosing the lowest bidder, late delivery of materials, escalation of local materialrnprices, ineffective project planning and scheduling, insufficient estimation of original contractrnduration, design errors made by designers, slowness in decision making, delay in site delivery,rnand unexpected surface & subsurface conditions. The findings from the survey result alsornshowed that the top ten determinant factors affecting cost overruns on water supply andrnsewerage construction projects in Addis Ababa are design change, price escalation of laborrnand materials, quantity underestimation, contractual claims, such as an extension of time withrncost claims, poor Contract management, omissions and errors in the bills of quantities,rnadditional work orders/extra items at ownerâ€™s request, inadequate project preparation,rnmonitoring, planning, and implementation, scope changes, and wastage/poor financial controlrnon site. Further, the main conclusion was that almost the majority of the water supply andrnsewerage construction projects in Addis Ababa city experienced extraordinary schedule delayrnand cost overruns to varying degrees due to the above-mentioned determinant factors. Thernaverage rate of schedule overrun is between 7.69% and 1208.89% of the original duration,rnand the average rate of cost overrun is between 0.72% and 8.11% of the original contractrnvalue. The projects were predominantly affected by the three key project players (client,rncontractor, and consultant). It is therefore, recommended, among other things, that allrnrelevant appropriate measures shall be put in place by the responsible parties (client,rncontractors, and consultants) to minimize schedule and cost overruns by implementing therngeneral project cost and schedule management practices, functions, procedures, tools, relevantrntechniques and using latest technology products from the beginning of the project initiation uprnto the final closure process