Running OS X and Windows simultaneously Using Bootcamp is it possible to run OS X and Windows simultaneously such that I can command-tab between the two?  I've run Windows using Bootcamp a few years ago but at that time a restart was required when switching between OS X and Windows.
Bootcamp is designed to allow you to change your operating system while the computer boots.

Nothing allows you to swap purely back and forth between operating systems that way. What you can do is use software that runs Windows in OS X with hardware virtualization, a huge step up from the software virtual machines of old.

There are three good options: Parallels, VMWare Fusion, and VirtualBox, the last of which has the advantage of being free.

If you want more technical details, particularly the ones that drove an edit to improve this answer, read the comments below.