1 Introduction

Blazars show flux variability from radio to $\gamma$-ray energies. Variability timescales are generally shorter at higher energies. Some of the flux variations in different energy regions might have a common origin, for example, simultaneous multifrequency observations have indicated correlations between $\gamma$- and radio emission (Teräsranta & Valtaoja [1996]) and between optical and $\gamma$-ray emission (Wagner [1996]; Bloom et al. [1997]). Compton Gamma Ray Observatory (CGRO) discovered in the early 1990's that radio-loud blazars (BL Lacs and HPQs) emit $\gamma$-rays at MeV- and GeV-energies. Some of them have shown remarkable flux variability at GeV-energies on timescales of a few days. Intensive monitoring will provide information on their variability and will help to study emission processes.

Due to inverse Compton-scattering, the optical photons in the beamed relativistic jet might be scattered to frequencies which are 10⁶ times higher than the original frequency. The new frequency should then be in the $\gamma$-ray range. Optical monitoring simultaneous with the $\gamma$-ray satellite pointings gives an opportunity to test how these optical variations correlate with variations in the $\gamma$-ray range.

The optical monitoring of blazars at Tuorla Observatory (University of Turku) started 20 years ago (Sillanpää et al. [1988]a, [1991]), being one of the oldest quasar monitoring programs in Europe still in progress.