This chapter reviews general understanding of the factors that control planform characteristics of rivers. It first introduces the major planform types – straight, meandering, braided, and anabranching – and discusses the distinguishing characteristics of each type. Environmental domains of each planform type, particularly in relation to stream power and sediment characteristics, are then discussed. The chapter examines transitions between planform states and the notion of whether channel planform changes over a continuum or is governed by thresholds that separate distinct domains of planform behavior. It also discusses recent work suggesting that the largest rivers in the world exhibit planform characteristics that differ from those for small rivers. The chapter serves as the foundation for detailed discussions of the dynamics of different planform types in subsequent chapters.