  I describe recent examples of phase transitions in four-dimensional M theory
vacua in which the net generation number changes. There are naive obstructions
to transitions lifting chiral matter, but loopholes exist which enable us to
avoid them. I first review how chirality arises in the heterotic limit of M
theory, previously known forms of topology change in string theory, and
chirality-changing phase transitions in six dimensions. This leads to the
construction of the four-dimensional examples, which involve wrapped M-theory
fivebranes at an $E_8$ wall.
