The Arp2/3 complex is a seven-protein assembly that is critical for actin nucleation and branching in cells. Here we report the reconstitution of active human Arp2/3 complex after expression of all seven subunits in insect cells. Expression of partial complexes revealed that a heterodimer of the p34 and p20 subunits constitutes a critical structural core of the complex, whereas the remaining subunits are peripherally located. Arp3 is crucial for nucleation, consistent with it being a structural component of the nucleation site. p41, p21, and p16 contribute differently to nucleation and stimulation by ActA and WASP, whereas p34/p20 bind actin filaments and likely function in actin branching. This study reveals that the nucleating and organizing functions of Arp2/3 complex subunits are separable, indicating that these activities may be differentially regulated in cells.