Algae, generally held as the principal primary producers of aquatic systems, inhabit all conceivable habitats. They have great ability to cope with a harsh environment, e.g. extremely high and low temperatures, suboptimal and supraoptimal light intensities, low availability of essential nutrients and other resources, and high concentrations of toxic chemicals, etc. A multitude of physiological, biochemical, and molecular strategies enable them to survive and grow in stressful habitats. This book presents a critical account of various mechanisms of stress tolerance in algae, many of which may occur in microbes and plants as well.