The show deals with a vast conspiracy of lies and cover-ups and alludes to the real-life corruptibility of government. Scully and Mulder often break the law to get the information they're looking for. However, the partners treat each other with care and kindness even if they often disagree.
Most episodes have scary elements, some with gruesome creatures and behavior. Some episodes include strong violence, including stabbings, gunfights, and physical violence -- some very graphic.
Some sexual tension between main characters, and some episodes deal with sexuality on a variety of levels (an alien shape-shifts from male to female after having sex, for example) while others don't address it at all. Rare allusions to unusual sexual practices, such as inbreeding.
Mild profanity, such as "damn," "hell," and "ass."