In this talk, I will present the bulk scale size dependent fracture toughness of a class of ultra lightweight, nano-architected metamaterials comprised of brittle consituents. Rapid progress in additive manufacturing methods has led to the creation of a new class of ultralight, stiff and strong 3D architected metamaterials comprised of a network of stretch-dominated, micro-scale unit cells. These materials, with only a fraction of their parent materials (ceramic, polymer, or metallic), possess extraordinary mechanical, electric and thermal mechanical properties not seen in their bulk constituents. The performance of these micro-architected metamaterials, spanning across multiple length scales, will ultimately be limited by their tolerance to damage and defects, yet an investigation of their fracture toughness has remained elusive. Here, we provide the fabrication and measurement of a class of different crack initiation modes in notched low density, hierarchical metamaterials. We find that, through hierarchical micro-architected features, the low density, stretch-dominated metamaterials can achieve simultaneously higher fracture toughness and strength: properties that are usually mutually exclusive. Numerical and scaling relationships are also reported that accurately capture the measured fracture toughness and strength values. These are then used to develop design maps for the optimal hierarchical architectures as functions of the density of the metamaterial and failure strain of the parent material.