We summarise results from flux density monitoring campaigns performed with the 100 m radio telescope at Effelsberg and the VLA during the past 15 yrs. We briefly discuss some of the statistical properties from now more than 40 high declination sources (δ ≥ 30°), which show intraday variability (IDV). In general, IDV is more pronounced for sources with flat radio spectra and compact VLBI structures. For 0917+62, we present new VLBI images which suggest that the variability pattern is modified by the occurrence of new jet components. For 0716+71, we show the first detection of IDV at millimetre wavelengths (32 GHz). For the physical interpretation of the IDV phenomenon, a complex source and frequency dependent superposition of interstellar scintillation and source intrinsic variability should be considered.