How did quantum fluctuations of the inflaton field provide the seeds for structure on cosmological scales? Did other fields play a role during inflation? How did their quantum fluctuations imprint on cosmological observables (e.g., non-Gaussianity) or survive as cosmological relics today (e.g., dark matter, matter/antimatter asymmetry)? How can these degrees of freedom be embedded into a compelling UV theory? By bringing together experts on particle physics and cosmology, we hope to develop a deeper understanding of these tough questions over the course of this 4-week MIAPbP meeting.