The first and most obvious avenue for future work is the continued monitoring of the radio sources mentioned here, to follow any evolution in the sources over an extended period of time. These observations will be important for all of the sources but especially important for PKS 1718-649 since they may further confirm the GPS nature of the source. This type of observation is also particularly important for the gamma-ray loud and gamma-ray quiet radio sources since apparent superluminal motions provide some indication of the importance of relativistic beaming. These observations have already been approved as future SHEVE experiments.
The value of monitoring sources over an extended period is illustrated through the work presented in chapter 5 on Centaurus A. In the particular case of Centaurus A, this work has indicated that very rapid evolution is taking place on the sub-pc-scale. The future monitoring of Centaurus A should be undertaken with a higher temporal frequency, so that the rapid evolution can be better sampled. These observations are currently underway (1996 February - 1996 June) at the VLBA with a frequency of 8.4 GHz and a temporal spacing of approximately one month.
A major opportunity for future investigations of the sources described in this thesis will be provided by the VLBI Space Observatory Programme (VSOP) space VLBI mission, which will be launched into Earth orbit in 1996 September. This mission will result in an imaging VLBI array with a maximum resolution of approximately 50 
as, at 22 GHz. One of the goals of the mission is to provide very high resolution images of the unresolved radio cores of AGN. This capability will be used to the best advantage for Centaurus A, the closest active radio source and hence the best target for the high resolution investigation of a radio core. The SHEVE team has been awarded VSOP time to investigate the Centaurus A radio core.
A major goal of the VSOP mission is to extend VLBI baselines to the extent where accurate high radio core brightness temperatures can be measured and used as an indicator of relativistic beaming. These observations perhaps represent the best opportunity to test models which attempt to explain the differences between gamma-ray loud and gamma-ray quiet AGN in terms of relativistic beaming. The observation of radio core brightness temperatures above approximately 10 
K give a direct estimate of the Doppler factor for relativistic beaming. The author is principal investigator on a successful VSOP proposal to investigate gamma-ray loud and gamma-ray quiet blazars in the Southern Hemisphere.