Careful timing of eclipsing binaries has shown that systems showing orbital period modulations on the order of ∆P/P ~ 10 over a period of decades are quite common. A striking example of such a system is Algol, for which the detailed observational record extends back over two centuries. Over this span of time, a graph of the time dependence of the difference between the observed times of eclipses versus the predicted times shows a feature (termed the "great inequality") with a full amplitude of 0.3 days and a recurrent time scale of centuries. Superimposed on this feature is a secondary modulation with a full amplitude of 0.06 days and a recurrent time scale of about 30 years. Orbital period modulations of similar amplitude are seen in other Algol binaries as well.