Engineering life to do our bidding is damn hard. It's made even harder (and more dangerous, though that's a story for another time) by the law of unintended consquences: living beings are incredibly complicated, and they mutate quickly and in unexpected ways, and we simply don't understand them all that well.

Enter synthetic biology, which we might call the practice of using bits of DNA ("bio-bricks") to build pseudo-organisms which can grow and act (even replicate) in more precisely-controlled ways -- creating "machines" which are not quite like anything found in nature, and yet clearly, in some basic ways, alive, or at least akin to the living. It is, in essence, a biotech way of hacking the law of unintended consequences.

This Scientific American article does one of the best jobs I've yet seen of explaining synthetic biotechnology.

The article stops short of exploring the dangers and pitfalls, the unintended consquences of a field of endevour designed to skirt around the unintended consquences of tinkering with the structure of life itself. Not too much precautionary principle here. Still, if you're interested in the future biotechnology (and if you're interested in the future, you ought to be, whichever side of the issue you tend to find yourself), this is required reading.