On-surface synthesis has been one of the hottest research fields in surface science in the last decade, owing to its great potential for bottom-up synthesis of functional molecules and covalent nanomaterials. Compared to classical in-solution chemistry, all of the on-surface reactions are done without solvent, thus very minimal byproducts and no limitation of solubility are involved. However, because of its typically required ultra-high vacuum conditions, where only limited catalysts can be used, a key challenge for on-surface synthesis is the precise control of the reaction pathway. Countless efforts have been made for controllable synthesis of target chemical structures on surfaces by distinct strategies. These strategies can be summarized under following aspects: 1) rational choice of surfaces; 2) template effects based on two-dimensional (2D) environments; 3) on-surface thermodynamic and kinetic controls; 4) the participation of chemisorbed nonmetal adatoms on surfaces. This report reviews the recent progress toward the control of on-surface synthesis and raises a series of questions at the end, which deserve further explorations in the future. Display omitted