Electron density irregularities in the equatorialD andE regions have been studied using ground-based and rocket-borne experiments. In this paper this subject is reviewed with an emphasis on the rocket-borne measurements of electron density and electric field structures and other parameters relevant for identifying processes responsible for generation of electron density irregularities. These studies have led to the identification of various causative mechanisms, in terms of plasma and neutral dynamical processes for producing irregularities. Based on these mechanisms, the irregularities are classified and discussed. There are many observations which have eluded explanation and hence call for sustained efforts.