|Quantitative spectroscopy of the solar atmosphere will be discussed and demonstrated with measurements obtained in the framework of international cooperations on the ground and by instruments flown on spacecraft. The necessary design, calibration and cleanliness requirements will be presented. Examples of such measurements are: Accurate wavelength determinations are needed for spectral line identifications. Doppler broadening of the lines provides information on ion temperatures and turbulent ion speeds. Dopplergrams allow helioseismic studies and Zeeman-Doppler imaging shows the line-of-sight component of the photospheric magnetic field. Plasma motions and oscillations can also be obtained from line shifts. The spectral distribution of the solar irradiance and its variability are important results of calibrated observations as is the integrated Total Solar Irradiance (TSI). Such data establish the radiation output of the Sun at the contemporary epoch. Spatial structures and their dynamics can be studied with the help of temporally resolved radiance and spectral radiance measurements, and line-ratio as well as emission-measure analyses yield plasma parameters in the solar atmosphere.