During the last decade multiwavelength observations of blazars have revealed many interesting patterns in their emission across the EM spectrum. In the present article we will review the time-dependent one-zone models and the models which advocate an acceleration and a radiation zone, and we will make some comparisons between them, especially in light of recent observations of the so-called TeV blazars.

We present an optical intraday variability (IDV) study of S5 0716+714 carried out over 52 nights. The source showed detectable variations (>0.05 mag) in 80% of the nights. Typical variation rates of 0.02 mag/hour have been found to last on average 4 hours, with the average rising rate faster than the decreasing one. A maximum rising rate of 0.16 mag/hour has been observed. If the luminosity variation is due to jittering in the jet direction, a comoving dθ/dt rate of ∼100 arcsec/hour is required, assuming for the jet θ ∼ 7° and Γ ∼ 14.

Very long baseline interferometry (VLBI) polarisation measurements provide information about the parsec-scale magnetic field structures in compact active galactic nuclei (AGN), as well as the densities of relativistic and thermal electrons in the radio emitting regions. This paper reviews the role of polarisation VLBI in studies of AGN variability on both relatively long and short (intraday) timescales.

The OVV blazar 3C 446 was monitored over seven months in 1997 with ground based telescopes and instruments on ESA's Infrared Space Observatory (ISO). The aim was to try to detect variations in the IR and see if these were correlated with optical variations. The object varied in the optical and near-IR during this period, but did not vary in the far-IR. Despite being a factor of ten weaker than in 1983, the optical-IR SED exhibited the same slope. The new far-IR observations from ISO allow us to determine the location of the turnover in the spectrum, caused by synchrotron self-absorption. It occurs just longwards of 100 μm.

We present a study of X-ray column density variability in Seyfert 2 galaxies. We show that variations in NH are observed in almost all the objects with multiple hard X-ray observations. Variation timescales (as short as a few months in several cases) are not in agreement with the standard scenario of a parsec-scale toroidal absorber. We propose that the X-ray absorber in Seyfert galaxies is located much nearer to the centre than previously assumed, on the broad line region scale. An extension of the model by M. Elvis (2000) can explain the observed variability. We also show preliminary results of NH variability search inside single X-ray observations, which suggest that variations can occur on timescales of a few 104 s.

We present a multifrequency monitoring campaign of PKS 2005–489, a bright and highly variable blazar. Simultaneous observations were completed over a 13 day period in the X-ray with RXTE, in the optical at the CTIO 0.9 m telescope, at TeV energies with CANGAROO-II, and at submillimetre wavelengths with SEST. Previous multiwavelength monitoring campaigns of PKS 2005–489 and other blazars have found complex flux and spectral variability behaviour, with different modes and timescales from days to hours and shorter.

PKS 2005–489 was observed in transition from a quiescent state into the early stages of a flare. A gradual increase in flux density was observed at optical and X-ray wavelengths during 19–27 August 2000, and a dramatic increase in X-ray emission was observed starting on 2 September 2000. Intraday and spectral variability were not detected during the campaign.