The process of pairs trading involves exhaustively matching and ranking pairwise stocks based on some prespecified measure of closeness; e.g., correlation, cointegration, sum-of-squared price difference. Pairs trading is popular for various reasons. It is simple to follow and execute. The pairwise portfolio can be nearly market-neutral, such that it does not require the comprehensive analysis of macroeconomic news. Since it is based on relative valuation, the actual worth of individual firms is not a pertinent consideration. The strategy is sufficiently flexible to accommodate various investment styles. Lastly, it does not evoke frequent intraday rebalancing, such that pairs trading can be automated to a certain extent and be cost-feasibly profitable. Despite its long history on Wall Street, pairs trading remains elusive in nature. The academic attention it attracts is modest compared to contrarian and momentum trading.