This chapter examines the characteristics and dynamics of river floodplains. The view of the floodplain as an integral part of the river system is developed, including the role of overbank flows and lateral movement of channels across floodplains in floodplain dynamics. Major depositional mechanisms on floodplains, including lateral accretion and vertical accretion, are reviewed along with erosional processes that can affect certain types of floodplains, resulting in localized scour or catastrophic stripping, in relation to distinct planform types. Characteristic sedimentological assemblages are introduced in relation to distinct planform types, including meandering, braided, wandering gravel-bed, and anastomosing rivers. Three-dimensional alluvial architecture of floodplains is highlighted and examples provided of how this architecture can be used to interpret floodplain alluviation history and changes in the depositional dynamics of river/floodplain systems over time. The chapter also examines the classification of floodplain types in relation to fluvial energy regimes and the factors controlling spatial variations in floodplain extent and volume.