This thesis contains results from a long-term campaign to monitor the behaviour of Be/X-ray binary systems at multiple wavelengths. The work presented in this thesis concentrates on the application of Be/X-ray binary observations to contribute to the understanding of the Be star phenomenon itself. I present studies of the X-ray behaviours of these systems - effectively using the Neutron Star as a probe of the Be star's envelope - and seek correlation between this and the behaviour of the Be star inferred from optical and infrared observations. I attempt to model the X-ray light-curves as accretion from a circumstellar disc, and investigate the validity of these techniques by considering recent claims that the orbiting neutron stars in Be/X-ray binaries perturb the Be stars' circumstellar discs. Finally, I present a study of Be/X-ray binaries in the Magellanic Clouds, identifying optical counterparts to a number of recently discovered systems.