Only the ones labeled microwave-safe. A lot of everyday plastic containers, like takeout tubs and margarine containers, aren't built for it.
Plastic that survives a trip through the dishwasher or freezer doesn't automatically survive a trip through the microwave — the concerns are different. Microwave heat can cause certain plastics to warp, leach chemicals faster, or break down in ways that don't happen at other temperatures.
The FDA regulates and tests containers specifically labeled microwave-safe, which is why that label is worth checking rather than guessing based on how sturdy a container looks.
The FDA requires manufacturers to test containers marketed as microwave-safe for how much, if any, chemical migrates into food under microwave heat, and to keep that within approved limits for the intended use. Look for the microwave-safe symbol (usually wavy lines) molded into the bottom of the container, separate from the recycling triangle and its number.
Vent the lid or leave it slightly open so steam can escape — a fully sealed container can warp or pop open from built-up pressure. Don't let plastic wrap touch the food directly unless the box specifically says it's microwave-safe plastic wrap, since direct contact under heat is a different scenario than wrap sitting an inch or so above the food.
For anything reheated more than a couple of minutes, very fatty or oily food (which gets hotter than watery food and can exceed what plastic is rated for), or food you're not sure about the container's history, glass or ceramic removes the guesswork entirely. It's a small habit that avoids the whole question and holds up to far more reheats over its lifetime — a set of glass containers with vented lids pays for itself quickly if you reheat leftovers often, since it works equally well for freezer storage, the fridge, and the microwave without the container ever becoming the weak link. Keep a couple of vented, microwave-safe lids on hand specifically for reheating, separate from the everyday lids you use for cold storage, so you're never tempted to reach for a takeout tub out of convenience when the right container is just as easy to grab.
The label is the deciding factor, not how tough the plastic feels. When you're unsure, moving food to glass or ceramic for two minutes is easier than researching a takeout container's plastic type.
Not much on its own — that number identifies the resin type for recycling, not whether it's microwave-safe. Always look for the separate microwave-safe symbol instead.
Generally no — most takeout containers aren't tested or labeled for microwave use, even if they held hot food originally. Transfer the food to a labeled microwave-safe container first.
Unlabeled or damaged plastic can leach more chemicals under microwave heat, which is exactly what microwave-safe testing is meant to limit. Labeled containers are held within FDA-approved migration limits.
Not automatically — BPA-free describes one specific chemical the plastic doesn't contain, not a general microwave-safety certification. Check for the separate microwave-safe label too.