According to the American Association of Marriage and Family Therapists (AAMFT) "Marriage and Family Therapists broaden the traditional emphasis on the individual to attend to the nature and role of individuals in primary relationship networks such as marriage and the family. MFTs take a holistic perspective to health care, they are concerned with the overall, long-term well-being of individuals and their families."
What does that mean? It means that even when working with an individual, I look for how things make sense in the context of their lives. What is going on in the family that may be contributing to the teenager acting out? How does your spouse's actions influence your own and vice versa. Everyone's behavior makes sense in context and MFTs look at the context, not just the behavior.