The strengths and shapes of spectral lines contain a great deal of information about the stars, and the line absorption coefficient plays a fundamental role here. The situation in this regard is similar to the effect the continuous absorption coefficient has on the shape of the continuum. Lines are more interesting, however, because several different physical effects can enter the structuring of the final absorption coefficient. Each of these has its own variation with wavelength across the line, that is, its own absorption coefficient. The main processes we consider in this chapter are: (1) natural atomic absorption, (2) pressure broadening, of which there are several, and (3) thermal Doppler broadening. The final combined absorption coefficient is the multiple convolution of these individual absorption coefficients. One of the remarkable results of these studies is that the natural atomic absorption and all the significant pressure broadenings have the same wavelength dependence in their individual absorption coefficients (with the notable exception of the hydrogen lines), namely the dispersion profile. This leads to a tremendous simplification, as we shall see, since the convolution of dispersion profiles is a dispersion profile with a half-width equal to the sum of the individual half-widths of the contributing processes. The thermal broadening, on the other hand, reflects the Maxwellian velocity distribution of the absorbing atoms and ions via the Doppler effect, so its wavelength shape is a Gaussian. The final absorption coefficient is therefore a convolution of the dispersion profile with the Gaussian profile.