AbstractIntraday variability in compact flat-spectrum radio sources has been intensively studied in recent years. For most IDV events the apparent brightness temperatures derived from the observed timescales are in the range of T_b,app ~ 10¹⁶⁻¹⁸ K. For extremely rapid variations, T_b,app can reach up to ~ 10²¹ K (e.g. Kedziora-Chudczer et al., 1997). Refractive interstellar scintillation may be the most likely extrinsic mechanism (Rickett et al., 1995; Qian, 1994a; Qian, 1994b). Especially for the case of extreme T_b,app (> 10¹⁸ K) RISS may be dominant (Dennet-Thorpe and de Bruyn, 2000). However, some IDV events with T_app ~ 10¹⁷⁻¹⁸ K show evidence for an intrinsic origin e.g. the correlated radio-optical intraday variations observed in the BL Lac object 0716+714 (Wagner and Witzel, 1995, Qian et al, 1996). It seems important to distinguish between IDV which is a phenomenon intrinsic to the compact radio sources and IDV which is primarily due to RISS. Multifrequency polarization and VLB I observations would be most helpful (Gabuzda and Kochanev, 1997).