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One of the oldest scientific topics in Equatorial Aeronomy is related toSpread-F. It includes all our efforts to understand the physical mechanismsresponsible for the existence of ionospheric F-region irregularities, thespread of the traces in a night-time equatorial ionogram – hence its name –and all other manifestations of the same. It was observed for the first timeas an abnormal ionogram in Huancayo, about 70 years ago. But only recentlyare we coming to understand the physical mechanisms responsible for itsoccurrence and its capricious day to day variability. Several additional techniqueshave been used to reveal the spatial and temporal characteristics of the F-regionirregularities responsible for the phenomenon. Among them we have, inchronological order, radio star scintillations, trans-equatorial radiopropagation, satellite scintillations, radar backscatter, satellite androcket in situ measurements, airglow, total electron content techniquesusing the propagation of satellite radio signals and, recently, radarimaging techniques. Theoretical efforts are as old as the observations.Nevertheless, 32 years after their discovery, Jicamarca radar observationsshowed that none of the theories that had been put forward could explainthem completely. The observations showed that irregularities were detectedat altitudes that were stable according to the mechanisms proposed. Abreakthrough came a few years later, again from Jicamarca, by showing thatsome of the "stable" regions had become unstable by the non-linearpropagation of the irregularities from the unstable to the stable region ofthe ionosphere in the form of bubbles of low density plasma. A problemremained, however; the primary instability mechanism proposed, an extended(generalized) Rayleigh-Taylor instability, was too slow to explain the rapiddevelopment seen by the observations. Gravity waves in the neutralbackground have been proposed as a seeding mechanism to form irregularitiesfrom which the instability would grow, but the former are difficult toobserve as a controlling parameter. Their actual role still needs to bedetermined. More recently, radar observations again have shown the existenceof horizontal plasma drift velocities counter streaming the neutral wind atthe steep bottom of the F-region which produces a fast growing instabilityfrom which a generalized Rayleigh-Taylor instability can grow. Themechanisms proposed would explain the rapid development of the large andmedium scale irregularities that have been observed, including some seenonly by radars. Nevertheless, a proper quantitative theoretical mechanismthat would explain how these irregularities break into the very importantmeter scale ones, responsible for the radar echoes, needs to be developed.This paper makes a selective historical review of the observations andproposed theories since the phenomenon was discovered to our currentunderstanding

Spread F – an old equatorial aeronomy problem finally resolved?