Mechanical metamaterials are a class of architectured materials which possess exceptional and tailorable mechanical and physical properties by engineering their internal lattice structures. The lightweight and multifunctional properties of metamaterials make them widely used in diverse applications, such as aerospace and biomedical engineering. However, manufacturing these geometrically complex structures has been a challenging task. The rapid development of additive manufacturing (AM) technology in this decade makes them possible. To fully explore the achievable mechanical properties of metamaterials, two research tasks should be performed systematically: (1) identification of lattice structures with optimal mechanical properties and (2) minimization of defects induced by AM processes. In this talk, our latest research progress on the development of truss- and shell-based lattice structures with superior stiffness and strength via analytical, numerical, and experimental methods will be presented. The geometric defects of selective laser melting fabricated structures, the influence of defects on mechanical properties, and compensation methods will be discussed. Finally, the applications of our proposed metamaterials in the fields of lightweight structural components, bone scaffolds, and electrode structures will be demonstrated, which completes the whole journey of metamaterial and structural development from design, manufacturing, and characterization towards applications.