A major theme of this book is the hypocrisy and corruption of society - at least society as portrayed in the series of mean little towns along the Mississippi river. In the earlier book, The Adventures of Tom Sawyer, there are certain nostalgic and idealising influences which paint an overall rosy picture of the town of St Petersburg. Here, however, such influences are notably lacking (except perhaps in the scenes set on Phelps’ farm, which do seem to lapse back into the tone of the earlier work). There is violence everywhere, deceit and greed.
Society’s wrongs are highlighted by the contrast with the youthful innocence of the narrator Huck, who often appears puzzled and repulsed by its ways. Its rules and conventions are seen to be generally at fault in this book. This ranges from comic instances such as Huck wondering why it's wrong for a boy to smoke when it's okay for a respectable woman like the Widow Douglas to take snuff, to the serious questions of slavery and injustice.
A related theme is that of freedom. Huck escapes into nature, first on Jackson's Island and then on the raft with Jim:
We said there warn’t no home like a raft, after all. Other places do seem so cramped and smothery, but a raft don't. You feel mighty free and comfortable on a raft. (chapter 28)
Huck, then, feels liberated by the raft and the river. All the same, he is not completely at ease during the journey, as he is tormented by the false notion implanted in his head by society that he has to turn the fugitive Jim in. Indeed, the book seems to show that a complete retreat from society is not possible.