The lagg of a raised bog is a transition zone where runoff collects from the ombrotrophic (rain-fed) bog and adjacent mineral soils. Distinct hydrological and hydrochemical gradients exist across the lagg zone, resulting in specific plant communities. Little research emphasis has been placed on the lagg zone in the past, with studies tending to focus on the more easily-defined bog instead. Recently, peatland researchers have begun to discuss the importance of the lagg to raised bog restoration. This paper reviews current knowledge on lagg zones, the function of this transition zone, some useful indicators to determine its location in the field, and argues that restoration of the lagg should be a key element in raised bog restoration.