What is the difference between when and while When" is used to refer to a defined period in time, an occasion or an event that has a sense of a concrete start and end, or "finishing", if you like. Examples: When the Romans invaded Britain, they met little resistance . You will be able to vote when you are 18 . While" on the other hand is more to do with the duration of that time period or event. It's used to refer to simultaneity, of something happening during something else. Examples: While Hitler invaded France, Britain built up its air force . I'll clean the windows while you wash the floors . There are some occasions where either word may be used and it doesn't make a lot of difference, but still there'll be some subtleties of meaning, dependant upon choice of word used. a) Bill said "I will watch the movie when you're asleep, Ted b) Bill said "I will watch the movie while you're asleep, Ted a) means that Bill will not start watching the movie until Ted has fallen asleep. It's the start of Ted's sleeping that is the important thing - it's the key specific event that Bill's waiting for so that he may start watching the movie . b) also means that Bill won't watch the movie unless Ted's asleep, but he'll do it at some undefined stage during Ted's sleeping. It's far less specific and more indefinite.