This volume is a collection of three essays relating to the pricing of securities in financial markets, such as stock markets, where a large number of individual securities are traded. Lead-Lag Effects in a Competitive REE MarketThis essay introduces a model of cross-security information aggregation. The model is essentially an extension of Chan (Journal of Finance, 1993) to the case of simultaneous auction markets where revealed information is correlated across securities.The model provides clear predictions of lead-lag effects between securities returns. Several of the model's predictions are confirmed empirically using data from the Paris Bourse. Other models of price formation, including the basic Chan model and nonsynchronous trading, are rejected as they cannot account for observed return patterns. Learning the True Index LevelThis essay extends the model of cross-security information aggregation by deriving implications for autocorrelation in index returns. Both time series and cross-sectional predictions are confirmed by empirical evidence from the Paris Bourse. In addition, the time series predictions are consistent with earlier, partly unexplained, empirical evidence from the US market. An Empirical Study of Index Return AutocorrelationThis essay studies return autocorrelation on the Stockholm Stock Exchange focusing on the relation between index returns and indvidual stock returns. It is demonstrated that the two return types have similar time series properties, and it is concluded that the causes of autocorrelation are the same in both cases. / <p>Diss. Stockholm : Handelshögskolan, 1997</p>