Address invitation to woman and her husband?

If you know the name of the husband, it is proper etiquette to address the mailing envelope to "Mr. and Mrs. [his name here]". On the inner envelope the addressing can be much more informal, with a simple "John and Jane", etc.

Another Perspective

This is the twenty first century. Women have names and an existence apart from their husbands. An invitation can be addressed to Margaret and William Anderson. Most etiquette books are decades behind the times.