From a dental health perspective, it's best to brush your teeth after a meal. The bacteria that live in your mouth are particularly good at metabolizing sugar and by brushing your teeth after a meal, you're removing the food for the bacteria to eat. These bacteria can result in dental carries (cavaties), which is obviously bad for your dental health, but also increases the likely hood of inflammation throughout your body. This is the reason gum disease is linked to so many other diseases.

As for before a meal, there's no reason you can't give yourself a thorough water rinse or even a toothpaste-less brush if you just want to "freshen up" before breakfast.

Bonus points: brushing afterwards also decreases the chance that you'll eat more (because you'll have the toothpaste taste in your mouth) and so brushing after meals is also a great way to thwart grazing throughout the day.

While I agree that brushing after breakfast is correct, I would just add that if you drink coffee or tea and are worried about staining, it is best to brush after a meal rather than before. That having been said, I always brush or at least rinse before breakfast as it makes my breakfast taste so much better.

It is important that you brush twice a day with a toothpaste containing fluoride. The best times are before breakfast and last thing at night before you go to bed.

Eating and drinking naturally weakens the enamel on your teeth, and brushing straight afterwards can cause tiny particles of enamel to be brushed away. It is best not to brush your teeth until at least one hour after eating.

It is especially important to brush before bed. This is because the flow of saliva, which is the mouth’s own cleaning system, slows down during the night and this leaves the mouth more at risk from decay.

So brush away in the morning and night. Try to limit yourself directly after meals.

I'd say brush when you get up (or at least rinse) and after every meal as well as before retiring to bed...

Brushing before eating breakfast has been proven to reduce the harmful plaque that causes tooth decay. Plus, you'll be able to taste your breakfast much better after brushing the germs out of your mouth and taste buds. Once you've eaten, just chew some sugar-free gum or swish your mouth out with water and you'll be just fine.

While many of the answers are correct there is a alternative theory out there. Brushing before eating will reduce the existence of biofilm. Biofilm harbors the harmful bacteria waiting to feast on your breakfest. If the biofilm is removed there is nothing for your food to stick to. Unless, of course, you have open embrasusres, deep pits and fissures, or crowded teeth where food will impact and be waiting for those little buggers when they rebuild.

Brushing throughly twice daily and flossing once a day is usually the best defense. Before or after a meal. It's a case of biofilm and preference.