The 11-year solar cycle modulates radiation hazards throughout interplanetary space. The galactic cosmic ray (GCR) background varies approximately inversely with the 11-year cycle, as defined by the traditional Wolf sunspot number, by nearly 30 percent over the course of the cycle; the background reaches minimum around the time of sunspot maximum and maintains a broad maximum around the time of sunspot minimum. Solar particle events (SPEs) are short, intense events that typically last a few days but produce particle fluxes much larger than the GCR background for brief periods of time. SPEs may occur at any time in the 11-year solar cycle, but historically have been more frequent during the years around sunspot maximum. Protection against radiation exposure during future exploration-class space missions depends on knowledge of the form and length of the solar cycle, both of which are extremely variable. Limitations in our ability to predict these quantities over long time scales may increase the complexity of radiation protection problems for exploration missions. This paper reviews the current limitations in predicting the solar cycle and the associated levels of SPE and GCR fluxes.