1 Introduction

Observing extragalactic radio sources, generally speaking quasars has, been increasingly popular during the last years due to the numerous space borne instruments dedicating a large share of their observing time to this field. Instruments like EGRET onboard the Compton GRO (gamma rays), IUE (ultraviolet), GINGA, ASCA, XTE and ROSAT (X-rays), ISO (infrared) and many others have prompted coordinated ground-based observations. Two observing windows can be covered from the ground: the radio window (below 300 GHz) and the optical window. Monitoring of extragalactic radio sources has been continued with the Metsähovi radio telescope since 1980, most of the observations being at 22 and 37 GHz. Our sample mostly consists of Northern flat spectrum sources which have at least once reached 2 Jy at 22 GHz (Valtaoja et al. 1992). At lower frequencies (5, 8 and 14 GHz) the Michigan group has been monitoring a larger sample, which also includes most of our sources, since 1965 (Aller et al. 1985). At still higher radio frequencies there are three monitoring efforts: our 90, 135 and 230 GHz monitoring of southern sources with the SEST-telescope (Tornikoski et al. 1996), the Ian Robson team working with the JCMT-telescope at 150, 230, 270 and 375 GHz (Stevens et al. 1994), as well as the IRAM-team at 90, 142 and 230 GHz (Reuter et al. 1997) and in references therein. At optical wavelengths there are several monitoring efforts concentrating on AGN, like the ones at Tuorla Observatory (Sillanpää et al. 1991), at Rosemary Hill Observatory (Webb et al. 1988), at Foggy Bottom Observatory, at Calar Alto Observatory by the Hamburg Quasar Monitoring Program (Schramm et al. 1994) and the Heidelberg group, at Asiago Observatory (Barbieri et al. 1977), at Perugia University Observatory (Fiorucci & Tosti 1996), at Torino Observatory (Villata et al. 1997), at Dodaira Station (Kikuchi 1988) and at the Yunnan Observatory (Xie et al. 1994). The Western Kentucky University has a new automated AGN monitoring program with two optical telescopes (Hackney et al. 1996). Typically the variations in AGN are slower at radio frequencies than at higher frequency bands and thus the sampling density does not have to be so high. A monthly sampling will usually be adequate for cm-observations. Below 3 mm you should have weekly observations to get most of the variations covered. At optical frequencies even faster sampling would be desired.