Recent focus on global climate change has elevated interest in understanding natural populations evolutionary adaptation to temperature and predicting how these populations may respond to temperature changes. This book addresses three aspects of adaptation to temperature focusing on Daphnia (Crustacea: Cladocera) as a model system. First, it discusses and examines the validity of the controversial assumption that can also be described by a proverb jack-of-all-trades is a master of none". Second, it examines genetic variation and heritability of thermal tolerance of three closely located Daphnia populations and provides predictions how these populations would adapt to a changing thermal environment. Third, it examines the effect of optimal/stressful temperatures on Daphnia juvenile survivorship and how such effects could