The book on sea ice ecology is the ecology of sea ice algae and other microorganism as bacteria, meiofauna, and viruses residing inside or at the bottom of the sea ice, called the sympagic biota. Organisms as seals, fish, birds, and Polar bears relies on sea ice but are not part of this biota. A distinct feature of this ecosystem, is the disappearance (melt) every summer and re-establishing in autumn and winter. The book is organized seasonally describing the physical, optical, biological, and geochemical conditions typical of the seasons: autumn, winter, and spring. These are exemplified with case studies based on author fieldwork in Greenland, the Arctic Ocean, and Antarctica but focused on Arctic conditions. The sea ice ecosystem is described in the context of climate change, interests, and effects of a decreasing summer ice extent in the Arctic Ocean. The book contains an up to date description of most relevant methods and techniques applied in sea ice ecology research. This book will appeal to university students at Masters or PhD levels reading biology, geosciences, and chemistry.