The Saga of Grettir the Strong by Unknown is an epic story that explores themes from Nordic folklore. The most prominent theme is the concept of justice. The titular hero journeys back to his homeland, which has become lawless. The concept of justice is based on different interpretations, but in this case is rooted in the idea of order. Without order, the fundamental basis of law, a society cannot function properly.

The theme of morality (or, more specifically, the idea of who is branded as a criminal) is also questioned. The protagonist is seen as an outlaw by his enemies, but this is only because he disrupts their ambitions of power. This is similar to the debate concerning the idea of a terrorist versus the idea of a freedom-fighter; the perception of a person's deeds is more complex than black and white thinking. An outlaw to some might be a hero to others.

Another theme is the concept of ancestral traditions. In Icelandic and Scandinavian culture, lineage is important. Grettir is a descendant of a famed Viking warrior, and this ancestry guides his principles. The multiple examples of violence in the saga illustrate how justice should be achieved by any means necessary. The violence also shows the survival-of-the-fittest mentality necessary to live in a hostile world. In this context, "strong" pertains to both physical and mental characteristics.