Stricktlydating, some are, & some are not, it should mention this on the base, or the packaging of your dish but please, remember that plastic leeches into your food when you microwave but also, I hope you will read my hub, & all the comments, on the effects of microwaving before you continue, to provide you valuable information & awareness to make up your mind from there onwards.
Most of those plastic containers will melt when heating up food in them. Always look on the the bottom for the microwave symbol or a statement saying, microwavable! I save my ice creams containers for freezing large quantities of soup!
I agree with Inspired to Write. I only microwave as a last resort. When I do, I do not use plastic. I use pyrex glass storage containers for storing food. If I heat something in the microwave, I use just plates, bowls, etc.
It's a good idea to limit exposure to plastic as it is hazardous. I've learned a lot on this topic in the last couple of years. I wrote a hub that discusses the impact of plastics on your body systems. Here's the link to it: http://fitmom.hubpages.com/hub/Hormones-Gone-Wild
Thank you fitmom, it is good to let others know about life's inventions that are supposed to aid us, so that people are not blinded thus, they can choose to research facts before deciding to make up their minds, especially, about using a microwave.