Language of thought theories fall primarily into two views. The first view sees the language of thought as an innate language known as mentalese, which is hypothesized to operate at a level below conscious awareness while at the same time operating at a higher level than the neural events in the brain. The second view supposes that the language of thought is not innate. Rather, the language of thought is natural language. So, as an English speaker, my language of thought would be English. My goal is to defend the second view. My methodology will see the project broken down into three major areas. First I will show that human thinking requires a language of thought, after which I will highlight some problems with assuming that this language is innate and hidden. Included in this section will be a small introduction to the compatibility problem. The compatibility problem offers some obvious difficulties for mentalese theories and these will be discussed. The next stage of the project will focus on evidence that can be put forward in support of the claim that natural language is the language of thought. Our most direct source of evidence comes from introspection, and this will play a dominant role in the discussion. The final part of the thesis will involve an examination of the principle arguments that have been put forward against the idea that natural language is the language of thought. My goal will be to show that these arguments do not entail the existence of mentalese, nor do they show that natural language is not the language of thought. I will provide answers to the arguments, and will explain the phenomena they point to in terms of natural language being the language of thought.