Abstract:We study the duality between M-theory on compact holonomy G 2 -manifolds and the heterotic string on Calabi-Yau three-folds. The duality is studied for K3-fibered G 2 -manifolds, called twisted connected sums, which lend themselves to an application of fiber-wise M-theory/Heterotic Duality. For a large class of such G 2 -manifolds we are able to identify the dual heterotic as well as F-theory realizations. First we establish this chain of dualities for smooth G 2 -manifolds. This has a natural generalization to situations with non-abelian gauge groups, which correspond to singular G 2 -manifolds, where each of the K3-fibers degenerates. We argue for their existence through the chain of dualities, supported by non-trivial checks of the spectra. The corresponding 4d gauge groups can be both Higgsable and non-Higgsable, and we provide several explicit examples of the general construction.