Injustice is seen in many ways throughout the novel. For the most part, we see injustice in the trial of Tom Robinson. From the beginning of the trial, Atticus knows that it isn't a matter of innocent or guilty, it is a matter of race. In Chapter 9, Atticus states, "The evidence boils down to you-did—I-didn’t. The jury couldn’t possibly be expected to take Tom Robinson’s word against the Ewells..." This quote captures the progression of the entire trial. Later, after the trial is over, our narrator tells us, "Atticus had used every tool available to free men to save Tom Robinson, but in the secret courts of men’s hearts Atticus had no case. Tom was a dead man the minute Mayella Ewell opened her mouth and screamed." (Chapter 25). Again, the theme of injustice is seen. The trial wasn't centered on Tom's actions, but Tom's color.
The idea of injustice is contrasted with the idea of fairness throughout the novel as well. Scout refers to many things as "unfair" early in the novel, not truly understanding the word. Calpurnia treats her unfairly, her teacher is unfair to reprimand her, Jem is unfair to her in excluding her, etc. However, she starts to understand the idea of "fair" when Miss Maudie talks about why Atticus doesn't shoot any more. She says that Atticus has an unfair advantage over most living things, and hasn't shot until he had to. As the trial progresses, Scout understands more and more the true definition of "fair," and is able to see the injustices of the world more clearly.