The central viewpoint of the book is to present and underline sea ice as a ecosystem in it's own right with clear pathways of energy and matter inside the ice and in exchange with surrounding water and atmosphere. The book is a description of the microorganism living inside the sea ice, their lifecycles, how they interact, and what mechanism they have develop to survive in this extreme environment. The book describes all the microorganisms living in sea ice: meio- and macrofauna, microalgae, bacteria, virus and fungi. Sea ice in a climate context is described in separate chapters with a focus on the ecological consequences of a diminishing summer sea ice extent in the Arctic. The book is written with a focus on graduate and Phd. students, and researchers with an interest in sea ice as a high-level introductory text with extended references with focus on all major aspects of microbial life in sea ice and embedded in a climate change context.