When a firm charges different customers different prices for the same product. For producers, the perfect world would be one in which they could charge each customer a different price: the price that each customer would be willing to pay. This would maximize producer surplus. This cannot happen, not least because sellers do not know how much any individual would pay. Yet some price discrimination is possible if an overall market can be segmented into somewhat separate markets and the equilibrium price in each of these markets is different, perhaps because of differences in consumer tastes, perhaps because in some segments the firm enjoys some market power. But this will work only if the market segments can be kept apart. If it is possible and profitable to buy the product in a low-price segment and resell it in a high-price segment, then price discrimination will not last for long.