The focus of this paper is on small and medium enterprises (SMEs)' access to credit and credit constraints, mainly in theoretical background and analytical framework. Firstly, the authors explore the behavior of demand for credit by small and medium enterprises and supply of credit by banks. Secondly, with literature survey, the paper is focus on determinants of credit constraints, including the characteristics of the lender and of SMEs, and their actions. Thirdly, the authors discuss the measurement of credit constraints, including direct methods based on information of firms' financing choices, or focused on firms' investment choices and production decisions, or based on qualitative information collected in interviews. Finally, the conclusions as policy implications and research implications are presented.