Sibling rivalry is the main theme of this novel. Peter, the oldest son of the Hatcher family, is often jealous of his younger brother, Fudge. His brother seems to get all the attention not only from his parents but from everyone who comes along. Peter thinks that this attention is given to Fudge because he is cute. People still think of Fudge as a baby. Because they think of him this way, they let Fudge get away with everything, like throwing temper tantrums, smearing restaurant walls with mashed potatoes, throwing popcorn at people when they go to the movies, and otherwise making Peter's life miserable.
Peter's patience and understanding, however, provide another major theme. Despite his jealousy, Peter often gives in to Fudge. He understands that Fudge, though maybe not still a baby, is still very young and has a lot to learn. Although Peter often gets angry at Fudge and the trouble he causes, Peter is willing to forgive him and give him another chance.
Although his family often resorts to tricking Fudge, his mom, dad, and Peter are sometimes very clever in their problem-solving tactics, which is another theme in this story. They all know that Fudge loves his brother and likes to mimic him. So they use this knowledge to get Fudge to do what they want him to do. If Fudge wants to have shoes just like Peter's, then they pretend that Peter is going to get shoes just like Fudge's. If they want Fudge to ride a tricycle around a filming studio for a commercial, they have Peter do it first and tease Fudge that they don't think he is big enough to ride the trike. The older family members find that even though they are tricking Fudge, their problems are solved and thus they avoid Fudge's loud screaming and crying bouts.
The power of laughter is a minor theme in this novel, with almost every chapter ending with the characters enjoying a good bout of giggles. Often is it Peter and his mom who laugh at the way they have solved their biggest problem in life, namely, Fudge. But Fudge often makes them laugh too. Fudge, when he is not eating flowers or messing up Peter's school projects, or throwing food on the floor, can be quite charming and also funny.