*Taken from the dictionary* Environment: 1. external conditions or surroundings, esp those in which people live or work 2. the external surroundings in which a plant or animal lives, which tend to influence its development and behaviour Circumstances: * 1. a condition of time, place, etc, that accompanies or influences an event or condition * 2. an incident or occurrence, esp a chance one 若要用中文大概地解釋的話，environment 是‘環境，生境’，circumstances 是‘情況，處境，狀況’。

Environment refers to: 1.The totality of surrounding conditions. he longed for the comfortable environment of his living room" 2.The area in which something exists or lives while circumstance may refers to: 1.A condition that accompanies or influences some event or activity 2.The set of facts or circumstances that surround a situation or event 3.Information that should be kept in mind when making a decision 4.Formal ceremony about important occasions "pomp and circumstance" cowboy...

Well.. mostly the environment is used to describe natural surrounding, when we talk about the air pollution the global warming the Hurricane Katrina,we may use environment there.On the other side, circumstance is more likely to be something that is related to human beings, like an accident a special occasion a unpleased or unsatisfied situation and so on. For more information, log on http://dictionary.reference.com/