The mammalian embryo exhibits a remarkable plasticity that allows it to correct for the presence of aberrant cells, adjust its growth so that its size is in accordance with its developmental stage, or integrate cells of another species to form fully functional organs. Here, we will discuss the contribution that cell competition, a quality control that eliminates viable cells that are less fit than their neighbours, makes to this plasticity. We will do this by reviewing the roles that cell competition plays in the early mammalian embryo and how they contribute to ensure normal development of the embryo.