Mitochondria possess a complex architecture with two membranes. The inner membrane is divided into two domains: the inner boundary membrane, which is adjacent to the outer membrane, and membrane invaginations termed cristae. Both domains are connected by tubular openings, the crista junctions. Recent studies led to the identification of a large protein complex that is crucial for establishing inner-membrane architecture. This mitochondrial inner-membrane organizing system (MINOS) interacts with protein translocases of the outer membrane that are functionally connected to the endoplasmic reticulum (ER)-mitochondria encounter structure. Here, we propose that MINOS forms a central part of an ER-mitochondria organizing network (ERMIONE) that controls mitochondrial membrane architecture and biogenesis.