The trusty microwave has endured lots of slander since its 1955 introduction. It's powered by tiny spacemen. It'll secretly kill your plants. It just zaps the nutrition right out of the food. But one accusation that's been a little tougher to shake is the rumor that putting plastics in it will give you cancer. That's because there's certainly reason to be concerned about nuking plastics -- it's just not quite that simple.
First off, you should only be reheating plastic that is labelled "microwave safe." That's pretty easy to find -- Tupperware has a whole section for that stuff on their site -- but if all else fails, you can opt for ceramic or glass instead. You run into big problems when you shove not-up-to-code stuff like plastic takeout containers in the microwave because, as the USDA notes, they not only warp or melt under the wattage, but they can potentially leak bad chemicals into your food. And the "bad chemical" most people are talking about is BPA.
Some studies suggest that BPA exposure can increase your risk of breast cancer, prostate cancer, and other health issues -- mainly if you're exposed prenatally or as a young child. But the scientific community hasn't quite reached a consensus, and you need to be careful about which study you trust. The concern is big enough that the FDA banned any baby bottles or sippy cups containing the chemical, but currently the group has ruled BPA safe for the "currently approved uses in food containers and packaging." So while it's a bit of a murky area, your Mom's Tupperware party probably isn't the insidious death trap chain emails would have you believe. Probably.