Blazars are a class of active galactic nuclei (AGN). Active galaxies are those that host compact sources of non-thermal radiation in their nuclear regions and are usually classified as being radio-loud or radio-quiet. The nature of the central source is ullcertain, but arguments based on energy budget suggest that accretion onto a supermassive black hole is the most probable scenario (Rees 1984). At present, there is no satisfactory theory that can explain the difference between radio-loud and radio-quiet AGN. The emission from radio-quiet AGN is regarded as being thermal in origin. Thermal re-radiation of the nuclear energy by dust in the host galaxy is thought to be an important process in these sources (Bregman 1990; Hughes et al. 1993). Radio-loud sources are dominated by non-thermal radiation which is almost certainly synchrotron in origin. Blazars are the most extreme example of radio-loud AGN and as such, provide the best opportunity to investigate further the emission mechanisms associated with radio-loudness. The term blazar (Angel & Stockman 1980) was originally applied to two groups of AGN which exhibit similar properties, namely BL Lacertae objects and optically violently variable (OVV) quasars. The BL Lac objects can be further split into radio selected BL Lac objects (RBLs) and X-ray selected BL Lac objects (XBLs). The difference between these two groups may only be due to the angleat which the jet (see below) is directed towards the line-of-sight of the observer. In this scenario the RBLs are more closely aligned (e.g. Gear 1994). The BL Lac objects studied in this work are all RBLs. In recent years, differences between OVV quasars and BL Lac objects have become apparent and these, together with the similarities that led to the original unification, will be discussed below. It is now more correct to assign the term blazar to a phenomenon rather than to a distinct class of AGN.