To a Married Couple
You need to include both names on the outer envelope. Guests rely on the outer envelope to determine who exactly is invited. For example:
Mr. William and Mrs. Melissa Simpson
Mr. and Mrs. William Simpson
555 Main Street
Roanoke, VA 22016
The Inner Envelope
On the inner envelope, lose the first names or the titles and last names if you're very close with the couple:
Mr. and Mrs. Simpson
William and Melissa
Formal invitations are always slipped into an unsealed inner envelope to be placed inside the outer envelope. They are addressed in a more informal fashion and include the names of all invitees at the address, including children. For example: Mr. and Mrs. William and Melissa Simpson, Rachel and Troy
To a Married Couple Uses Different Last Names
List the person you're closest with first on the outer and inner envelope. If you're similarly acquainted with both, list them in alphabetical order.
Mr. William Simpson and Mrs. Melissa Stewart
To an Unmarried Couple Living Together
Like a married couple, both names should be included on the envelope, but in this case, each name gets its own line.
Mr. William Simpson
Ms. Melissa Stewart
555 Main Street
Roanoke, VA 22016
Again, go for alphabetical order if you know the couple well. If you're primarily friends with only one member of the couple, it's completely acceptable to address the outer envelope to that person and include "and Guest" or that person's name on the inner envelope:
Mr. Simpson and Ms. Stewart
Mr. Simpson and Guest
To a Same-Sex Couple
Use the same rules as you would for any other unmarried or married couple. If the couple is married or lives together, list them on the same line:
Ms. Ellen Deer and Ms. Jackie Park
Or simply list their full names on the same line without titles.
Ellen Deer and Jackie Park
On the inner envelope, use titles and last names:
Ms. Deer and Ms. Park
Or you can skip titles and simply address it to their first names.
To a Married Woman Doctor or Two Married Doctors
If a woman uses her maiden name professionally and socially, the outer envelope should read:
Dr. Mary Flowers and Mr. Allen Treece
Or, if she uses her husband's name socially:
Dr. Mary and Mr. Allen Treece
If both parties are doctors, you can address the outer envelope:
The Drs. Treece
Drs. Mary and Allen Treece
The same format is followed for other distinguished titles, such as reverend and honorable.
To Children and Families
Younger guests can be included on the inner envelope of their parents' invitation by their name(s) or "and Family" -- although the latter is easier to misinterpret.
Mr. and Mrs. Roger Pulitzer
Lilly, Alan, and Christopher
Mr. and Mrs. Roger Pulitzer and Family
To Children 18 and Older
They should receive their own invitations, although siblings over 18 can be sent a joint invitation, in which case the outer envelope should include their names alphabetically on individual lines:
Miss Samantha McCoy
Miss Wanda McCoy
Mr. Sebastian McCoy
If you don't include "and Family" or each child's name, you're implying that children are not invited. However, do not be surprised if some guests still assume their children are welcome. If you are concerned this will happen with your guests, ask your immediate family and bridal party to help spread the word that the wedding will be adults only.