This account summarizes predominately our recent endeavors in developing CO gas-free carbonylation reactions and the application of carbonylation reactions in the synthesis of heterocycles. Mo(CO)<sub>6</sub>, aldehydes, DMF, formic acid and its esters were employed as greener CO sources, and a series of palladium-catalyzed gas-free carbonylation reactions, including reductive carbonylation, amino- and alkoxycarbonylations, as well as carbonylative coupling reactions have been developed. Besides, we developed a series of carbonylation-based domino reactions for the rapid construction of heterocyclic compounds. 1 Introduction 2 Green Carbonyl Sources 2.1 Mo(CO)<sub>6</sub> as the CO Source 2.2 DMF as the CO Source 2.3 Formic Acid and Formates as the CO Sources 3 Carbonylative Synthesis of Heterocycles 3.1 Insertion of One CO Molecule 3.1.1 Intramolecular Nucleophilic Cyclization of Acyl Palladium 3.1.2 Intermolecular Nucleophilic Cyclization of Acyl Palladium 3.1.3 Cyclization through Nucleophilic Substitution 3.2 Insertion of Two CO Molecules 4. Conclusion