Accounting conservatism is one of the most important properties of financial reporting. The goal of this article is to gain a better understanding of accounting conservatism. We explicate the evolution of conservatism over its long history. Accounting conservatism is indispensable because the main parties of a firm demand conservatism to mitigate agency costs. Various methods are used to measure accounting conservatism, which include balance sheet measures, income statement measures and earnings/stock return relation measures. Empirical research into accounting conservatism has flourished over the last two decades and we focus on the cross-sectional and time-series variations in conservatism. We conclude that accounting conservatism is important and cannot be excluded from accounting standards.