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The dynamics of the Mesosphere and Lower Thermosphere (MLT) at polar and middle latitudes are investigated using VHF meteor radars. High-frequency gravity-wave activity and momentum fluxes are examined using Skiymet meteor radars based at Esrange, in Arctic Sweden (68°N, 21°E) and Rothera, Antarctica (68°S, 68°W). The 8-hour tide is investigated at middle latitudes using a meteor radar based in Castle Eaton, UK (52°N, 2°W).A novel technique for studying high-frequency gravity-waves with meteor radar is developed and applied to data from the radars at Rothera and Esrange. This technique is used to investigate the activity of gravity-waves with periods between about 5 minutes and 2 hours and horizontal scales less than about 400 km. A strong semi-annual cycle is revealed with solsticial peaks and equinocial minima. This behaviour is explained in terms of the critical-level filtering imposed by the stratospheric winds. Strong inter-hemispheric differences are also revealed. The theory is advanced that this is the result of the smaller winds speeds in the northern hemisphere in spring (due to the more rapid break-up of the stratospheric vortex).Variances calculated using this new technique are used to investigate interactions between the high-frequency gravity-wave field and tides and planetary-waves. Clear modulations of the gravity-wave field at tidal and planetary-wave frequencies are shown and reveal a relationship between the phase of the tide and the maxima in gravity-wave activity. It is proposed that the likely mechanism for this modulation is critical-level filtering by the tidal winds, acting on an anisotropic gravity-wave field. Zonal and meridional variances and momentum fluxes are calculated using data from the radars at Rothera and Esrange. Monthly-mean variances confirm the semi-annual seasonal cycle and reveal higher meridional activity over Rothera, particularly in summer. Again this is explained in terms of interactions with the mean winds as the waves ascend to the MLT. Zonal and meridional momentum fluxes reveal a somewhat more variable seasonal cycle, which is clearer over Rothera and indicates that at heights of about 90 km and below, momentum flux has a north-west direction in winter and south-east in summer. Data from the radar based at Castle Eaton is used to demonstrate that the 8-hour tide is of significant amplitude in both the mid-latitude and polar MLT. Climatologies reveal a seasonal cycle with a maxima in amplitude in autumn and a secondary maxima in winter. An interesting result is that the seasonal behaviour varies with latitude and the secondary winter-time maximum seen over the UK is not present in the Arctic. Analysis of hourly-mean amplitudes reveal possible interactions between the 8-hour tide, the 12- and 24-hour tides and planetary waves. An analysis based the 17 years of available data indicates a weak relationship between solar activity and the 8-hour tide. Tidal amplitudes tend to increase while phase tends to decrease as solar activity increases.