Solar irradiance variability has been studied from measurements of total solar irradiance at 1 AU, the solar ''constant'' S0, since the late 1960s. Power spectra of time series from satellite and balloon experiments show that the irradiance varies, with periods of minutes to several hours and with amplitudes of parts per million to a few hundreths of a percent. Part of this variance is due to coherent pressure and, possibly, gravity oscillations as spherical harmonics of the sun. On time scales of days to months most of the variance is a modulation of the solar irradiance by photospheric features related to solar activity and reaches amplitudes of a few tenths of a pecent. Because of its origin the variance in this range changes by up to 1 order of magnitude with the solar cycle, as shown by comparison of the power spectra from 1980 and 1984/1985, respectively. The first evidence of a persistent long-term change comes from satellite data, which show a downward trend of S0 since 1980 by about 0.02% per year. The S0 determinations between 1967 and 1970, compared with the modern ones show a statistically significant increase during that period of similar magnitude. Both trends together could be part of a 22-year cycle related to the magnetic cycle of the sun. ¿ American Geophysical Union 1987