The concept of linearization stability arises when one compares the solutions to a linearized equation with solutions to the corresponding true equation. This requires a new definition of linearization stability adapted to Einstein's equation. However, this new definition cannot be applied directly to Einstein's equation because energy conditions tie together deformations of the metric and of the stress-energy tensor. Therefore, a background is necessary where the variables representing the geometry and the energy-matter are independent. This representation is given by a well-posed Cauchy problem for Einstein's field equation. This book establishes a precise mathematical framework in which linearization stability of Einstein's equation with matter makes sense. Using this framework, conditions for this type of stability in Robertson-Walker models of the universe are discussed.