Using data sets on orders, order packages, quotes, trades and market-limit orders, the thesis investigated several aspects of the microstructure of the Saudi Stock Market (SSM) under the computerized trading system, ESIS (Electronic Securities Information System). The thesis studied the interaction between the order book and order flow, limit order execution, trading by limit versus market orders, order performance, and the information content of newly submitted orders. The findings provide new evidence for several issues, and have important implications for the design of the trading mechanism on the SSM.