In recent years, the Nigerian Stock Exchange (NSE) witnessed an unprecedented growth in market capitalization, membership, value and volume traded. By December 2007, the All-Share Index has grown massively to 57,990 from 1,113 in January 1993. This rising interest in investment opportunities in the NSE raises questions about its efficiency. Why do we care if a stock market is efficient or not? The stock market acts an intermediary and channels funds from savers to firms for investment purposes. An efficient stock market is a ‘well-functioning’ market, where prices of stocks represent their fair value. This book examines the efficiency of the NSE using a scientific method that can be replicated by stakeholders of stock market such as investors, investment analysts, fund managers, regulators, and students of the stock markets.