This book was developed to investigate and compare the efficiency within gulf cooperation council (GCC) based Islamic and conventional banks. The analysis was conducted over a sample of 49 banks for the period of 2006-2015. The study was divided into two stages of analysis; the first stage was conducted to estimate and analyze the efficiencies of the sample using data envelopment analysis approach (DEA). The second stage was developed to assess the bank-level and country-level determinants of the efficiency through random effect estimation approach with heteroscedasticity-corrected standard error. First stage of analysis suggested that Islamic banks (IBs) tend to be more technically efficient than conventional banks (CBs) due to their managerial efficiency in utilizing their resources and minimizing their waste of inputs.