The metabolism of one-carbon (C1) units is essential to plants, and plant C1 metabolism has novel features not found in other organisms-plus some enigmas. Despite its centrality, uniqueness, and mystery, plant C1 biochemistry has historically been quite poorly explored, in part because its enzymes and intermediates tend to be labile and low in abundance. Fortunately, the integration of molecular and genetic approaches with biochemical ones is now driving rapid advances in knowledge of plant C1 enzymes and genes. An overview of these advances is presented. There has also been progress in measuring C1 metabolite fluxes and pool sizes, although this remains challenging and there are relatively few data. In the future, combining reverse genetics with flux and pool size determinations should lead to quantitative understanding of how plant C1 pathways function. This is a prerequisite for their rational engineering.