Mitochondria continually fuse and divide to yield a dynamic interconnected network throughout the cell. During apoptosis, concomitantly with permeabilization of the mitochondrial outer membrane (MOMP) and cytochrome c release, mitochondria undergo massive fission. This results in the formation of small, round organelles that tend to aggregate around the nucleus. Under some circumstances, preceding their fission, mitochondria tend to elongate and to hyperfuse, a process that is interpreted as a cell defense mechanism. Since many years, there is a controversy surrounding the physiological relevance of mitochondrial fragmentation in apoptosis. In this review, we present recent advances in this field, describe the mechanisms that underlie this process, and discuss how they could cooperate with Bax to trigger MOMP and cytochrome c release. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.