This book addresses the prediction of the Water Quality Index (WQI) at Perak River, State of Perak, Malaysia, using a fuzzy multiple linear regression model to tackle the uncertainty in the measurements of six key parameters – dissolved oxygen (DO), biological oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (SS), pH value (pH) and ammoniacal nitrogen (NH3-NL). Given its scope, the book is suitable for graduate students, researchers and water quality scientists.