ature produces biomineral‐based materials with a fascinating set of properties using only a limited number of elements. This set of properties is obtained by closely controlling the structure and local composition of the biominerals. We are far from obtaining the same degree of control over the properties of synthetic biomineral‐based composites. A contributing reason for this inferior control is our incomplete understanding of the influence of the synthesis conditions and additives on the structure and composition of the forming biominerals. In this review, we provide an overview over the current understanding of the influence of synthesis conditions and additives during different formation stages of CaCO3, one of the most abundant biominerals. We relate this influence to the structure, composition, and properties of resulting CaCO3 crystals and summarize currently known means to tune these parameters. Throughout the review, we put special emphasis on the role of water in mediating the formation of CaCO3 and thereby influencing its structure and properties, an often overlooked aspect that is of high relevance.