This book examined the linkage between exchange rate variability and currency Substitution in Nigeria. Specifically, the study tested for the existence of currency substitution and attempted to gauge its magnitude in Nigeria. Also, causality between currency substitution and exchange rate volatility in Nigeria was investigated. Subsequently, the study analyzed the implications of currency substitution and exchange rate volatility for monetary policy in Nigeria. The study covered a period of 17 years –1986(1)-2004(4). Quarterly time series data collected from the International Financial Statistics published by the International Monetary Fund(IMF) and Central Bank of Nigeria Statistical Bulletin were used for the analysis. The time series properties of the variables were determined using the Augumented Dickey Fuller(ADF) test and the Phillip-Perron Z-test.The study adopted the unrestricted portfolio balance model, incorporating exchange rate volatility within the framework of the Vector Autoregression(VAR) technique.This was complemented with Autoregressive Conditional Heteroscedasticity(ARCH) model to determine the volatility or otherwise of exchange rate in Nigeria.