This will be the 27th meeting in this series of annual international multidisciplinary meetings, created in 1989 by Jean Tran Thân Van and which has in the past covered many topics in physics, astronomy and biology. All sessions take place in the Château of Blois, a beautiful renaissance castle which has housed many French kings, and notably François 1st. The 27th Rencontres de Blois on "Particle Physics and Cosmology" (Blois2015) will emphasize the increasing interplay between high energy accelerator based physics and cosmology. The conference will consist of plenary sessions for invited in-depth oral presentations (review talks) and contributed papers, in the form of relatively short oral papers. We will aim to achieve a balance between review talks, provocative talks given by recognized specialists, and shorter contributions. Special emphasis is being placed on active participation by younger researchers and post-docs. Parallel sessions are foreseen, and are being organised as the need arises.