The Arecibo Observatory will soon complete a major instrumental upgrade which will provide improved capabilities for observations of the upper atmosphere. As in the past, Arecibo capabilities center on 430-MHz incoherent scatter radar (ISR) measurements of the ionosphere made locally or in conjunction with other national and international incoherent scatter facilities; the upgrade will add the capability for simultaneous two beam incoherent scatter observations and will extend plasma line measurements to ¿15 MHz. Aeronomical studies may also be performed using a 46.8-MHz coherent scatter radar, the feed of which is coaxial with the 430-MHz line feed, the newly refurbished medium-frequency (MF) radar facility, and a new digital ionosonde. The original airglow laboratory continues to house the Fabry-Perot interferometers, Ebert-Fastie spectrometer, and tilting filter photometers used for observations of mesospheric, thermospheric, and exospheric airglow, while a new laboratory provides a permanent home for the lidars used for resonance fluorescence observations of atomic metal layers in the mesopause region and for Doppler Rayleigh measurements of the upper stratosphere and lower mesosphere. Finally, the recently upgraded high-frequency, high-power transmitter facility has performed active aeronomical and plasma physical studies alone and in conjunction with other Arecibo and visitor-supplied instruments. Observations performed as the upgrade nears completion have already provided a first look at what lies ahead in areas as diverse and fundamental as the structure and dynamics of the turbopause, the properties of upper atmospheric tides, the electrodynamics of the E and F regions, the dynamics of light ions in the topside, and the physics of plasma turbulence. Examples of these and other observations are presented, and the opportunities for future investigations are discussed. ¿ 2000 American Geophysical Union