The (2bar 1bar 1)langle111rangle twinning energy landscape of BCC Mo is determined using the density functional theory for embryos containing 2 to 7 layers. The 2-layer embryo is metastable, whereas the 3- and 4-layer ones are unstable. Layer-by-layer growth starts at 5 layers. The twin boundary formation and migration energies are found to be 607 mJ/m2 and 40 mJ/m2, respectively, indicating that twin partial dislocations have wide cores and high mobilities. The stress to homogeneously nucleate an additional partial loop on the boundary of a sufficiently thick twin is only 1.4 GPa; this implies that once a deformation twin reaches critical thickness, which we estimate to be 6 layers, subsequent growth in thickness is easy.