The 21st century has seen tremendous progress towards understanding the elementary constituents of matter, the forces that bind them, and the organizing principles that unite them. Concerted experimental and theoretical efforts have culminated in the discovery of the Higgs boson and the observation of gravitational waves, while also excluding countless proposed new phenomena. Today the central questions of particle physics are sharper than ever – from the nature of dark matter and dark energy, to the patterns of flavor and unification, to the origin of mass and the mechanism of electroweak symmetry breaking -- but answering them will require both the energetic synthesis of existing methods and the genesis of entirely new ones.

Fortunately, opportunities for progress abound. In seeking new ways to address its central questions, particle physics has built bridges to numerous adjacent fields. Within particle physics, the past decade has seen extraordinary progress in both formal and computational theory, transforming our understanding of quantum field theory and our ability to extract predictions from it. There is an unprecedented opportunity to nucleate further progress by bringing together theorists from across the field to share perspectives and techniques. This field-spanning program in particle theory is aimed at focusing new developments in formal, computational, and phenomenological theory on the central questions of particle physics, and using those questions to drive further theoretical progress in turn. Beyond accelerating progress towards long-standing goals, the union of diverse perspectives and methods may well lead to entirely new questions to animate the particle theory enterprise.