The early days of VLBI are discussed, with emphasis on the fifteen experiments performed in the U.S. and Canada in 1967. Some key technical ideas are described that influenced the development of the technique, including digital fringe rotation, bandwidth synthesis, spectral phase referencing, phase closure and self-calibration, and water vapor radiometry.

The first VSOP satellite, HALCA, was successfully launched on 12 February 1997, ushering in a new era for VLBI astronomy. This paper will give a brief overview of the mission, satellite and the on-going In-Orbit Checkout of the mission elements.

The Caltech-Jodrell Bank VLBI surveys of bright extragalactic radio sources north of declination 35° were carried out between 1990 and 1995 using the Mark-II system, achieving images with a resolution of about 1 mas at 5 GHz. The CJl survey (together with the older “PR” survey) includes 200 objects with 5 GHz flux density greater than 0.7 Jy; the CJ2 survey includes 193 flat-spectrum sources with 5 GHz flux density greater than 0.35 Jy; and we have defined a complete flux-density limited sample, CJF, of 293 flat-spectrum sources stronger than 0.35 Jy. We summarize the definition of the samples and the VLBI, VLA, MERLIN, and optical observations, and present some highlights of the astrophysical results. These include: (1) superluminal motion and cosmology; (2) morphology and evolution of the “compact symmetric objects” (CSOs); (3) two-sided motion in some CSOs; (4) the angular-size-redshift diagram; (5) misalignment of parsec-scale and kiloparsec-scale jets.

I review the current paradigm for nonthermal emission from jets in blazars. The evidence for relativistically flowing jets seems to be more compelling than the case for incoherent synchrotron and self-Compton emission in these objects. I discuss some of the important aspects of current observations of rapid variability at radio frequencies, and stress that one must be careful when calculating the Doppler factor that saves us from the inverse Compton “catastrophe.” I discuss the possibilities for the nature of the VLBI components and present evidence that, at high frequencies, the core is a standing shock wave (Mach disk), while the superluminal components are propagating shocks. I expect that high-frequency and space VLBI will resolve many of the major outstanding issues.

I summarize the observational evidence which suggests: (1) that many of the compact radio sources associated with QSOs and related objects have been ejected from active galaxies so that their redshifts are not measures of their distances, and (2), that the energy emitted by compact sources is not gravitational in origin, but arises through creation processes in regions of galaxies with very strong gravitational fields.

By combining VLBI and total flux density data it is possible to get an indirect “look” at the innermost, unresolved core regions of AGN and to estimate physical parameters such as the intrinsic brightness temperatures, Lorentz factors and viewing angles, unobtainable by either VLBI or multi-frequency flux monitoring alone.

We report new results from high frequency (22-86 GHz) VLBI monitoring observations of selected blazars. These Gamma-bright sources show pronounced correlated flux density variations over the full electromagnetic spectrum (radio to Gamma-ray bands). From our high-angular resolution images (0.1-0.2 mas), we find increasing evidence for a tight correlation between this activity and the production of new jet components. Here we present results for the 3 sources PKS 0528+134, 3C 273, & 0836+710.

We provide a table of 42 blazars which we expect to be robust identifications of EGRET sources. The table includes the lensed source, PKS 1830–211 which we identify with a previously unidentified EGRET source.

We present here 3 epochs of 3 and 7 millimeter wavelength VLBI observations and 2 epochs of lower frequency VLBA imaging of the gamma-ray blazar NRAO 530. These observations document the evolution of the parsec scale jet in this source during the brightest flare in 3 decades. New jet components were created during the flare and are probably related to an increase in gamma-ray activity. The components travel at superluminal velocities, further confirming the connection between superluminal sources and gamma-ray blazars. The rapid evolution of the source makes tracking of components difficult. It appears that either components significantly decelerate or that there is rapid cooling and acceleration of elections. We may be identifying structure due to standing shocks. The jet is bent on all scales between 100 μarcsec to 10 arcsec. The results indicate the ability of 3 mm wavelength imaging to probe the nuclei of blazars rapidly and reliably.

In the past few years EGRET observations of extragalactic radio sources demonstrated the presence of blazars at high energies and supplied informations about their enormous luminosities. Questions arose whether outbursts at these high frequencies can be linked to structural changes in the radio regime and the need for VLBI monitoring became obvious. In the past two decades geodetic VLBI measurements gathered this kind of data.

We analyzed λ =3.6 cm observations with high time-resolution (up to once a month) from the geodetic IRIS campaigns for the γ-ray active quasars PKS 0420-014 and PKS 0528+134. In both sources superluminally moving jet components could be traced over time periods of several years on curved trajectories. Both sources exhibit component ejections preceding the time of an observed gamma-ray flaring.

Photons spanning over 17 decades in energy have been detected from Markarian 421. The discovery of >100 MeV photons by the EGRET instrument on board the Compton Gamma Ray Observatory led to the discovery of >300 GeV photons by the Whipple group using the air Čerenkov method. In May 1996 the source underwent an unprecedented outburst at these energies, inspiring observations with the VLBA to determine whether such high energy flares are associated with changes in the milliarcsecond structure of this BL Lac object.

The parsec scale radio jet of 3C 279 presents a number of superluminal features that move along straight trajectories with constant velocities. The position angles P.A. of these trajectories, projected in the plane of the sky, and the velocities βobs of the individual components are different. We interpret the differences in the these velocities as differences in the angle between the jet and the line of sight and apply the model of a precessing beam to the data. All the geometrical parameters of the precessing jet and the Lorentz factor γ of the relativistic particles are determined. The model predicts the behavior of the Doppler factor δ as a function of time and we verify that its maximum value occurred at the epochs in which strong optical and X-ray flares were observed.

We study the generation and evolution of superluminal components in relativistic jets through relativistic hydrodynamical and emission simulations of a square-wave perturbation in the jet velocity.

We propose that the Fanaroff & Riley division between class I and class II radio sources has its origins in the coronae of magnetized accretion disks around black holes. The “magnetic switch”, an effect that critically controls the speed of jets produced by such disks, can explain both qualitative and quantitative differences in the properties of the two classes. These include why morphology and jet speed are functions of radio power and why the division between the two classes is a strong function of galaxy optical luminosity.

We discuss the feasibility and prospects of broad–band spectral imaging based on quasi–simultaneous multi–frequency VLBI observations.

We have undertaken VLBI observations of 8 Southern Hemisphere EGRET radio sources. Using our data as well as data obtained from the literature we have examined the difference in radio properties between gamma-ray loud and gamma-ray quiet radio sources. In particular, we find no evidence that gamma-ray loud radio sources lie preferentially in sources with straight radio jets as has been suggested.

VLBI observations of the high-redshift quasar 1338+381 reveal a source with two well defined components that are stationary and possibly, a weaker component that moves in between at a very high superluminal speed.

We present observations of the FRI radiogalaxy 3C 66B made during a joint EVN + MERLIN VLBI session in May 1995. The jet is mapped from the 5 mas scale up to about 3 arcmin. The newly mapped VLBI jet shows essentially two elongated blobs in the direction of the brightest kpc jet (NE). We also present the first results obtained for a two-fluid model for VLBI jets.