The bid-ask spread in securities markets is a key factor affecting liquidity. Early models used daily transaction prices to estimate the spread, but with increased transparency in the 1990s, more accurate models were developed. These models decompose the spread into components like adverse selection and order processing. Two main approaches are used: one based on covariance properties and the other on trade direction indicators. Both approaches aim to estimate how order behavior impacts the spread. By comparing these models, researchers can better understand the factors influencing bid-ask spreads in financial markets.