In December 1990 a new IRI handbook was published by NASA's National Space Science Data Center (NSSDC) describing in detail the International Reference Ionosphere 1990. Shortly thereafter, the IRI-90 software was released on tape, diskette, and computer networks. This paper is intended as an inventory of the most important IRI activities up to 1990 and as a starting point for the next improvement cycle. It summarizes the work and studies that led to IRI-90 and provides an overview over this latest version of the model. Shortcomings and limitations are pointed out and ways of overcoming them are discussed. Priorities are suggested for the list of work items that the IRI group has to tackle in the future. High on the wishlist are major improvements at high latitudes and inclusion of magnetic storm effects. This first paper deals with the electron density; a follow-on paper discusses plasma temperatures, ion composition, and ion drift.