This thesis presents a study of some of the optical and infrared properties of blazars. A review of previous observations of blazars is given. The evidence for bulk relativistic flows in quasars and blazars is also discussed. The observational study has two parts. First a series of infrared and optical polarization measurements of blazars are presented. Second a study of the duster environments of blazars is reported. The study of the duster environments of blazars was formulated as a test of a unified scheme. Deep CCD observations were obtained for a sample of blazars and quasars with extended radio structure. The unified scheme predicts that these are distinguished only by their relative orientation. The strength of the cluster environment was parameterised by he amplitude of the quasar -galaxy spatial correlation function B₉q. The evaluation of this quantity is subject to large systematic errors as a result of the uncertainty in the galaxy luminosity function. Previously published data had to be re- evaluated so as to be comparable with the data presented here. No evidence was found for a difference in the environments of blazars relative to those of extended quasars. A previously reported correlation between B₉q and redshift was confirmed, but cannot be distinguished from an apparent relationship due to a correlation between B₉q and radio power since radio power is known to be correlated with redshift.