Quantum field theory (QFT) is the formalism that underlies modern particle and condensed matter physics. Standard perturbative methods in QFT have been extraordinarily successful in explaining physical phenomena involving weakly-interacting quantum fields. On the other hand many fundamental phenomena, including phase transitions and nuclear interactions, are described by strongly coupled QFTs for which perturbative techniques are insufficient and a rigorous, predictive theoretical formulation is lacking. Heuristic arguments indicate that a full non-perturbative formulation of QFT must include extended degrees of freedom (a prototypical example being the flux tubes that bind quarks inside the nucleus). My proposal describes a novel approach for studying extended objects in a wide range of QFTs, based on two recent conceptual breakthroughs: first, my research on a special class of theories (the six-dimensional SCFTs) has brought to light a rich algebraic structure that captures the properties of its stringlike excitations; and second, new developments in mathematics and physics point to the existence of a vast generalization of this structure, which is perfectly suited to describe the extended objects of a much wider range of QFTs. This program is organized along three directions: analyze the families of QFTs that can be studied by string-theoretic and geometric methods, and gradually uncover the algebraic structures that describe their extended degrees of freedom; exploit these algebraic structures to obtain novel principles that govern the dynamics of strongly-interacting QFTs; and determine the new mathematical structures that arise from the combination of the geometric and algebraic description of the extended objects. An ERC starting grant will allow me to undertake this ambitious project whose pursuit will lead to a much deeper understanding of extended degrees of freedom, or their role in QFT, and of the mathematical structures that describe them.