The current regulatory environment based on the ICH guidelines encourages a systematic and science-based approach in the pharmaceutical development, required by the “Quality by design” concept. This methodology implies that the quality of a product must be designed instead of assayed in the final dosage form. For this purpose, a deep knowledge of the factors affecting the quality of the product is needed to establish the design space. This design space is limited by critical points of the formulation whose knowledge is essential in order to develop a robust dosage form. This papers deals with the main critical points that must be taken into account in the design of solid dosage forms such as inert and hydrophilic matrices as well as controlled released systems based in new biopolymers. The influence of factors such as the particle size or the rheology of powders in these critical points has been analysed. Moreover, in silico simulation software has been employed to elucidate the release mechanism leading to unexpectedly low critical points in sustained release matrices prepared with two new polyurethanes.