A recent usage thread turned to the old question of the passive voice. Many people recommend that you turn the passive voice -- "The action was done by this person" (the object was verbed by the subject) or just "The action was done" -- into the active voice, "This person did this action" (the subject verbed the object).
This is generally good advice. Passive voice often makes writing less direct and thus less forceful: "Passive voice should be avoided by you" is worse than "Avoid the passive voice." It also sometimes conceals responsibility, as in the famous "Mistakes were made" used as a substitute for "We made mistakes."
But when it comes to writing, unwise editors often turn good general advice into a bad categorical rule. So it is here: "Generally avoid the passive voice" is good, "never use the passive voice" is bad.
In particular, if your discussion focuses more on the object than on the subject (the actor), it's often better to use the passive voice, which has a similar focus. If you're writing about the substance of the USA Patriot Act, for instance, the passive sentence "The Act was adopted shortly after the September 11 attacks" may be better than the active "Congress adopted the Act shortly after the September 11 attacks." The passive voice properly focuses the discussion on the Act, where you want it to be, rather than on Congress, which is not terribly relevant to your thesis. (Of course, if you were writing about Congressional decisionmaking related to the Act, "Congress adopted ..." may be exactly right -- but again the point is to choose the voice that fits what you want to emphasize, not to mechanically make everything active.)