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In many plant species, seed production is synchronized among individual plants across years.  This synchrony, known as mast-seeding in wild plants and as alternate bearing in crop plants, leads to fluctuations between years with high and low seed production at the population level. The fitness benefits of synchronous reproduction have been well studied, but the proximate causes of synchrony remain mysterious.  In this talk, I review global patterns of mast-seeding in wild and crop plants, and discuss two possible causal mechanisms of masting.  The first, known as pollen coupling, is an endogenous feedback that can lead to periodic or chaotic dynamics of seed production.  The second, known as the "delta-T"  model, is a simple external environmental cue that also leads to periodic forcing, even when the environmental conditions are not periodic.  Both models have some empirical support, in different plant species.  It is not surprising that a beneficial trait could arise in different ways in different species.  However, these different mechanisms of synchrony have different implications for how plants are likely to be affected by changes in the environment.