The purpose of this book is twofold: on one hand, it gives a mathematical perspective of Daphnia which comprises studies focusing on introducing size-structured population dynamics models of Daphnia and their unification, as well as developing and analyzing mathematical models of size-structured population dynamics of several species of Daphnia. These include proving the well-posedness of the model and related regularity properties, determining the steady states and proving the principle of linearised stability and studying the (in)stability of the steady states. The mathematical perspective of Daphnia is written at a research level and it is intended for researchers in applied mathematics, differential equations, biomathematics, mathematical biology, mathematical ecology as well as graduate students in the above mentioned fields of research. The necessary background for reading the chapters is the knowledge of ordinary differential equations, partial differential equations, integral equations and functional analysis. On the other hand, this book also provides a biological perspective of Daphnia by focusing on topics such as: the swimming behaviour, inducible defense mechanisms in order to avoid predation, sexual reproduction, the use of Daphnia as a classroom organism and the role of Daphnia in ecotoxicological evaluations of contaminants. In addition, it contains several video movies attached to some chapters. The biological perspective of Daphnia is written at a research level and intended for researchers in biology, chemistry, ecology and environment as well as graduate students in the above-mentioned fields of research.