History, or the Undo/Redo stack, is an important element of Meshmixer. One design decision we made early on was that we wanted to have very fine-grained History. This means that each parameter change inside a Tool becomes a checkpoint in the History, and hence you can Undo/Redo through parameter changes. We use the standard system hotkeys:

Undo: Ctrl+z (windows), Cmd+z (mac) 
Redo: Ctrl+y (windows), Shift+Cmd+z (mac)

However, one drawback of this fine-grained undo/redo is that you do have to step back through all those parameter changes. If you made a lot of them, or each (automatic) recomputation is even a little bit slow, it can get very frustrating. So, we also have a coarser level of Undo/Redo, which skips all these individual parameter changes, and only stops at Object changes:

Undo (Skip Tools): Alt + Shift + Left-Arrow (windows and mac)
Redo (Skip Tools): Alt + Shift + Right-Arrow (windows and mac)

Finally, we have a separate Undo/Redo stack for camera controls. You can step through this History using the parentheses keys: "(" and ")"

Memory Usage

One complication of working with high-resolution meshes is that to support Undo, we often have to store large amounts of data - sometimes entire copies of the mesh! This can eat up a lot of memory. So, if you find your computer bogging down after working in Meshmixer for a while (particularly while 3D sculpting), you might find that if you save a .mix and re-open it, performance will improve significantly. You will lose your Undo history, though.