The aims of this book are to investigate the role of the market structure in determining bank performance of the banking industry in the six GCC countries. This book consists of two stages. The first stage estimates the GCC banking industry's concentration. The second stage focuses on four different hypotheses that are explaining the relationship between market structure and performance using the Structure-Conduct-Performance (SCP) model. The emphasis of the second stage is: first, to analyse the relationship between market structure and banks' profitability and then it seeks to assess the relevance of the (SCP), the Relative-Market-Power (RMP), and the Efficient-Structure (ES) hypotheses in explaining the performance of the banking industry in GCC countries. Third, it tries to test the existence of the "Quiet Life Hypothesis" in these markets. Evaluating commercial banks overall performance and monitoring their financial condition is important to depositors, owners, potential investors, managers and, of course, regulators.