There is little to add to the post directly before this response as it is most comprehensive and thoughtful.  However, within the context of the psychological theme please consider, too, the role of conscience in the human individual.  As has been mentioned, guilt is, indeed, one of the emotions that Raskolnikov experiences.  But, as he seeks to surpress this sense of guilt, Rodion Romanovich Raskolnikov finds doing so increasingly difficult as he searches the eyes of Porfiry Petrovich, the unorthodox police inspector who is more absorbed in criminal psychology than in standard police procedure.  The examination of Dostovesky into the conscience of Raskolnikov is what is key to the theme of this most intriguing novel.  For, as great a theoretician as the student Raskolnikov is, he is not of a criminal mind, the sociopathic individual who feels no guilt.

It is hard to define just one overall theme and/or meaning of any Dostoevsky novel.  They are far too lengthy and complex for a simple answer.

This novel has a basic crime/murder plot.  While the question of "Who is the killer?" is already known by the reader - the suspense is built instead by "What is going to happen to Raskolnikov?"  Through this plot - Dosoevsky manages to intertwine several related themes.

The first is the psychology of crime.  After the main character commits murder, the book deals with all the different emotions he experiences, from guilt, to justification, to courage, to despair, to terror, to empowerment.

Another theme is the basic idea of isolation, alienation, or loneliness.  Raskolnikov's primary character trait is one of self-centeredness and superiority.  Throughout the story he pushes away the people who are trying to help him.  In his mind, he is a "superman" who is superior in society, therefore he is above societal rules.  This "superman" theory is the third major theme presented in the book.  It presents the question of is it okay to break the law (or a moral code) if it could possibly end in something great?

Like the rest of Dostoevsky's novels, Crime and Punishment is basically about psychologically intense (and somewhat twisted) characters and how they perceive the world.  I believe the author's main purpose in this type of genre is to present political, religious, and social questions that the average reader would otherwise never ask - and certainly be reluctant to otherwise examine.

To me, there are two major themes that are closely connected to one another.

The first is that all people are subject to the law of God.  No one is above the law.  There is no one who truly is a "superman" like Raskolnikov thinks he is at the beginning of the book.  We see this especially in Raskolnikov's dream in the Epilogue.

The second theme is the possibility of atonement and forgiveness.  We see a couple of different attitudes towards this.  Svidrigailov concludes that he cannot be forgiven, that he cannot atone.  Therefore, he kills himself.  Raskolnikov decides (with Sonya's help) that he can atone.  He ends the novel having found forgiveness and peace.