Depends on your subject, and the university. In CS departments (restricting to CS depts because that is what I am familiar with), the advisor will not let you defend unless you are ready, and the committee is already familiar with your work, and is not necessarily going to use your thesis as an introduction to their understanding. They already know the pieces well, and will have no difficulty navigating your thesis how ever you write it.

The final question is how broad your conclusions are. Are they relevant to a wide circle of people? -- For example, a new algorithm to sort numbers. If so, start with the basics. If on the other hand, if the thesis is about a particular optimization on a particular compiler technology, then you can assume that the reader is familiar with that compiler technology already.