Dicer is a specialized ribonuclease that initiates RNA interference (RNAi) by cleaving double-stranded RNA (dsRNA) into small RNA fragments about 22 nucleotides long. Here, we present the three-dimensional structure of human Dicer bound to the protein TRBP at approximately 20 A resolution determined by negative-stain electron microscopy (EM) and single-particle analysis. Our analysis reveals that the Dicer-TRBP complex is an L-shaped molecule with a long edge of 150 A and a 100 A extension on one end. A surface trench runs the length of the long edge of the molecule, defining a putative dsRNA-binding site. Docking the crystal structure of Giardia Dicer, which represents the nuclease core of human Dicer, into the EM map suggests two possible overall molecular architectures for human Dicer. These results offer insights into the structure of Dicer proteins found in multicellular organisms and provide a conceptual framework for understanding the initiation of RNAi.