Due to their high designability, covalent organic frameworks (COFs) are attractive candidates in many different applications. However, COF synthesis is still poorly understood and mainly trial and error based. In this review, we pave the way to a better understanding of COF chemistry and the synthesis of high-quality COFs. To achieve this goal, the main challenge in COF synthesis, the crystallization problem, is explained. Additionally, mechanisms for boroxine and imine COF formation are proposed. Afterwards, several approaches to increase COF quality are discussed: addition of modulators, slowing down monomer addition, improvement of used catalysts, study of solubility effects and interfacial synthesis, application of exchange reactions and finally linker specific observations. The discussed techniques can be seen as a toolbox usable for the synthesis of high-quality COFs.