Interacting systems abound at every level of biological organization, from the molecular to the population level. For example, molecular interacting systems consist of genes, their transcripts (mRNAs), proteins, and small molecules; their interactions include gene transcription, protein translation,protein–protein interactions, and chemical reactions. At the other extreme,interactions among animals include predation, competition, and symbiosis.A fundamental goal of biology is to understand why biological systems behave the way theydo. One promising avenue towardthis goal is to studyhow interacting biological systems at each level determine the emergent behavior at the next level. Emergence has been defined and studied in various contexts (Hopfield, 1982; Crutchfield, 1994; Kivelson and Kivelson, 2016; Kooreh-davoudi and Bogdan, 2016; Zheng, 2021); in this Element, emergence refers to the collective behaviors ofsystem components that arise from their interac-tions in a nontrivial way. For example, cellular decisions and phenotypes arise from the interactions of numerous molecular components. Similarly, interac-tions among cells determine how multi-cellular organisms develop and how tissues and organs function; interactions among individuals form the basis of social communities; and interactions among species underlie ecological com-munities. However, not every interaction plays the same role in determining the behavior ofthe system as a whole.