This paper reviews the effects of the ionosphere on radio astronomjcal observations, what we can learn about the ionosphere from radio interferometry, and a procedure to correct for these effects. This study analyzes the results obtained from observations of celestial point soUl.ces with the Westerbork Synthesis Radio Telescope, WSRT, in the Netherlands from the period 1970-1991. The main conc1usions are: 1) A1though seasona1 effects are c1ear, the occurrence and "strength" of ionospheric irregu1arities show no dependence on solar activity. 2) Assuming that the frequency of occurrence of ionospheric disturbances in Spring and Autumn are similar, Ihe "ionospheric" Winter starts on day 348 ± 3 and ali seasons last for three months. 3) Travelling ionospheric disturbances, TIDs, occur most frequently during daytime in Winter periods. 4) The propagation parameters of these travelling ionospheric irregularities and their periods indicate that these belong main1y to the c1ass of medium sca]e TIDs. 5) Radio interferometry is a powerful tool to locate irregularities causing scintillation and to determine their dimensions. 6) The occurrence of non-periodic irregu1arities is, however, not a function of time of day. 7) The daily variation in the amplitude and frequency of occurrence of the TIDs suggest that the generation of gravity waves may be caused by winds and tides in the lower thermosphere/mesosphere. On the basis of the availab1e data, a definition of a "disturbance measure" indicating to what extent the ionosphere is "quiet" is proposed. Procedures to correct for ionospheric effects and an eva1uation of the different methods to obtain information on the ionospheric e1ectron content are reviewed in sections 8 and 9, respectively.