Polyketides are a class of natural products with highly diverse chemical structures and pharmaceutical activities. Polyketide cyclization in Type II polyketide synthase (PKS), as promoted by the aromatase/cyclase (ARO/CYC), helps diversify aromatic polyketides. How the ARO/CYC  fosters highly specific cyclization was not well understood. The crystal structures and functional analyses of three PKS ARO/CYCs will be presented. The ARO/CYC possesses a helix-grip fold characterized by the presence of a large, solvent-accessible interior pocket. Co-crystal structures and docking simulations help visualize how the ARO/CYC interior pocket promotes specific folding of a linear polyketide and catalyzes intramolecular aldol condensation. Mutations of pocket residues, characterized by newly developed enzyme assays, further validated that the interior pocket of ARO/CYC is critical for polyketide cyclization. The chemical insights gleaned from this work pave the foundation towards defining the molecular rules for the ARO/CYC cyclization specificity, whose rational control will be important for future endeavors in the engineered biosynthesis of novel anticancer and antibiotic aromatic polyketides.