Such incidents, which seem like setbacks, are really just part of the process. "Until a child is three and hasn't had an accident for six months, he's not officially potty trained," says Edward Christophersen, Ph.D., professor of pediatrics at Children's Mercy Hospital, in Kansas City, MO.
Much of this has to do with other milestones he's achieving around the same time, especially the ability to play independently. Kids this age can get engrossed in an activity and simply don't notice the urge to poop or pee until it's too late. Others might realize they need to go but choose not to simply because they don't want to stop what they're doing.
This is also the time when many kids acquire a new sibling, which means potty mishaps may well have to do with an unconscious need to divert Mom's attention from the baby. Other changes that can trigger such behavior: a new sitter, a new house, a parent's stepped-up travel schedule.
But just because your toddler has a good reason for wetting his undies doesn't make it easier to deal with. Some guidelines:
Avoid using negative pressure, such as informing your child that "big boys don't wear diapers" -- this will just make him feel bad.
Don't set impossible goals -- like promising a treat if he stays dry for an entire week, for instance. You'll only set him up for disappointment if he doesn't succeed, and it's likely that he won't.
Help him remember to go. When he's hard at play, interrupt him for potty breaks. Tell him you're going to go too and need some company.
Draw as little attention to accidents as you can. Clean him up and send him on his way with a hug and some encouraging words. Then remind yourself that it won't be long before he's clean and dry all the time.