The Department of Physics and Astronomy at Manchester is one of the largest and most active physics departments in the UK.We have a long tradition of excellence in both teaching and research, and have interests in most areas of contemporary research. TheDepartment has a strong presence in a number of Manchester-based centres for multidisciplinary research: The National Graphene Institute, the Photon Science Institute, the Manchester Centre for Non-Linear Dynamics, and the Dalton Nuclear Institute. In addition, the Jodrell Bank Observatory in Cheshire is a part of the department. Work on theoretical physics is concentrated in four main areas: complex systems, quantum descriptions of matter and its interactions with light, nuclear physics, and particle physics. Brief summaries of these are given below; more information can be found on the webpages for each group. Quantum Theory of Light and Matter The Quantum Theory of Light and Matter group employs a broad range of numerical and theoretical methodologies spanning from first-principles calculations to quantum field theory approaches, as well as from tensor networks to analytical master equations to explore a diverse array of subjects. Noteworthy focuses encompass the study of quantum materials and their novel properties such as topological phase transition, superconductivity and quantum confinement, theories relating to quantum transport in low dimensions and mechanically distorted systems, emergent phenomena in strongly coupled non-equilibrium systems, quantum thermodynamics, quantum noise, and the dynamics of open quantum systems. The team maintains strong collaborative ties with experimental groups within the Department, as well as with the National Graphene Institute, which significantly contribute to the advancement of these theoretical pursuits. Nuclear Theory The research interests of the Nuclear Theory Group range from low-energy nuclear structure to the frontier where nuclear and particle physics overlap. We focus on 'fundamental' approaches to nuclear physics, linking it to quantum chromodynamics, and have particular expertise in the areas of effective field theory and microscopic many-body theory. Current particular interest include: the responses of nucleons and light nuclei to external fields (being probed with Compton scattering in experiments at Mainz and Duke Universities), and the origins of nuclear forces. Particle Theory The fundamental properties of matter are studied by the theory members of the Particle Physics Group. The Group has particular expertise in almost all aspects of Collider Physics phenomenology, Quantum Chromodynamics, in the Physics of the Early Universe, in Higgs and Neutrino Physics and in Physics Beyond the Standard Model. Our projects are often focused on aspects of theoretical physics that can be tested in ongoing or future experiments on colliders and non-accelerator physics, and in cosmological and astrophysical observations. The connections between particle physics and cosmology are also being explored in collaboration with members of the Jodrell Bank Observatory for Astrophysics.