Knowledge of the interstellar extinction is of great importance for the study of the properties of planetary nebulae. An estimate of the interstellar reddening for an individual nebula may be obtained by means of three independent methods. In the first method the observed Balmer decrement is compared with the theoretical decrement (case B of Baker and Menzel). The second method consists of a comparison of the observed ratio of Balmer and Paschen intensities of lines, arising from a common upper level, with the theoretically predicted ratio. Finally, for thermal planetary nebulae with well-determined radio spectra, the optically thin part of their spectra can be used for a determination of interstellar reddening. Comparison of the radio flux from an optically thin nebula with the observed Hβ flux (in absolute units) allows us to evaluate the amount of extinction. Radio observations may thus give the interstellar extinction with high accuracy. It may especially be noted that the Balmer-decrement method, which is widely applied, gives satisfactory accuracy only if highly accurate H-line intensities are used. In all other cases it gives the most uncertain results, since its wavelength base-line is small. One must also take into account the fact that the Hα-line intensity is often not measured.