On June 6, 1959, following the birth of Charlie Brown's sister Sally, Snoopy remarks that he has no brothers or sisters, and is an "only dog." This was later contradicted in the strip from May 5, 1965 in which Snoopy wonders what happened to his various brothers and sisters and Snoopy was later said to have been one of a litter of eight puppies.
One by one five out of Snoopy's seven siblings started to appear in the comic strip.
- Spike - First appeared August 13, 1975. He lives in a desert outside Needles, California. Spike was the first one of Snoopy's sibling to appear, and appears in the strip the most frequently out of all of Snoopy's siblings.
- Belle - First appeared June 28, 1976. She is Snoopy's sister and has an unnamed teenage son.
- Marbles - First appeared September 28, 1982. He is the smart one in the family.
- Olaf- First appeared January 24, 1989. He is the family misfit and was originally referred to as "Ugly Olaf".
- Andy - First appeared February 14, 1994. He is a shaggy dog and always appears with Olaf.
Most of the family seem to be in on Snoopy's World War I Flying Ace fantasy. Spike is often shown in the trenches as an infantry grunt and Belle appears as a Red Cross nurse for both brothers.
In the strip from June 18, 1989 Snoopy's father is shown receiving a Father's Day card. He comments that "all eight of them signed it". However, only six of his offspring (Snoopy, Spike, Belle, Marbles, Olaf and Andy) appeared in the strip. The other two siblings are Molly and Rover, TV-exclusive characters from Snoopy's Reunion.