Radar operating in the VHF band, at least, are capable of detecting unusually strong echoes from the mesopause region during summer. These polar mesospheric summer echoes (PMSE) have been reported mostly from radar situated in arctic regions, but midlatitude observations are not uncommon. First, the origins of PMSE are reviewed to give a background for what follows. The widely held assumption is that the presence of such echoes is related to the low temperatures found only at and around the summer mesopause, and given this assumption, we identify the times, from scales of 1 year to 1 hour, and geographic locations at which PMSE are expected to be observable. Other timescales, climatic to turbulent, are discussed qualitatively, and the whole study is presented as a planning tool for observations of the mesosphere which might have to take PMSE into account. ¿ American Geophysical Union 1996