There are a number of themes explored by Harper Lee in her novel, To Kill a Mockingbird. Among them:
LOSS OF INNOCENCE. This theme primarily involves the three children--Jem, Scout and Dill--who are exposed to a number of life-altering experiences over the course of the novel.
RACIAL PREJUDICE. Although the Tom Robinson trial is the prime example, there are other obvious examples of the intolerant attitudes by white Maycomb citizens toward their African-American neighbors. The church group, who pretends to offer aid to the Mruna tribe in Africa while scorning their own black neighbors, is just one such case.
COURAGE VS. COWARDICE. The children show great courage in standing up for their beliefs under duress, while Boo Radley shows his own bravery when he comes to the rescue of the children. Bob Ewell is the most obvious example of the coward, a man who preys on children in order to seek revenge against Atticus. The jurors, who refuse to accept the evidence before them at the trial, are another example.
KNOWLEDGE VS. IGNORANCE. The author seems to group Scout's various teachers among the ignorant members of the town. Miss Caroline tries to forbid Scout from reading with her father, and Miss Gates spouts her defense of the Jews in Germany while spewing her own bigotry against Maycomb's African-American community. Dolphus Raymond explains to Dill that he is too young to understand all the hatred around him, but that he will understand it better when he becomes older.