Select Query For Active Personell During Certain Period

I would like to make a query which is based on a table that holds information on employees (past and current)

I take the fields:

Name
Entry Date
Exit Date


and now would like to write a query where I can select a start and end date and the outcome should then be the "active" employees during this time.
for example between 1-7-2007 and 31-7-2007

I can not get my head around the criteria as the dates lie in 2 fields.

The exit date field can be empty or have dates that lie outside the range (see above) like the end of a contract at 30-10-2007

Hope someone can give me a tip to get me on my way.