Delays are the most common and costly problem in construction projects. The Owner assigns a Supervisor (OS) - either an individual Supervisor Engineer (SE) or a Consultant Organization (CO), one of OS role is to ensure that the project remains on schedule and protect the Owner from delays and associated claims. However, there are no existing means of determining the OSs' effectiveness in performing this task. This book developed a model to assess the effectiveness of both SEs and COs regarding construction project delays. The model assists COs in selecting SEs and preparing training plans. It also assists Owner to select their COs. According to this model, Libyan SEs are currently deficient in Communication skills; Obtained scientific, technical and legal knowledge; and Knowledge in dealing with claims. COs are deficient in terms of motivation, and quality systems. This work also investigates causes of delay in construction projects in Libya. The most frequent delay causes in Libyan are: delay in owner payments to the contractor; long administrative routines; instability of material prices; and frequent adjustments and additional work ordered by owner.