The purpose of this paper is to quantitatively examine the evolution of the informal economy over the past four decades. The study used the currency demand approach as analytical framework for the assessment. The findings suggest that there has been an upward trend in the size of the informal economy as a proportion of the officially recorded GDP. For instance, the size of the informal economy as a proportion of the official GDP estimates increased steadily, from 14% in 1960 to 18% by 1977. The proportion fell thereafter and started picking up again from 1983 to a new high of 30% between 2003 and 2004. The outcome of the study has policy implications particularly for the design of effective monetary and fiscal policy and the selection of appropriate policy instruments.