In the experimental sciences, new theories are developed by applying the scientific method to produce results which are accurate, reproducible and reliable. This involves testing the experimental setup to show that it is working as designed and thoroughly documenting the progress of the experiment. Results will not be trusted unless the experiment has been carried out to a suitable standard. In computational science, we should aim to apply the same principles. Results should only be trusted if the code that has produced it has undergone rigorous testing which demonstrates that it is working as intended, and any limitations of the code (e.g. numerical errors) are understood and quantified. Unfortunately, this can be quite challenging. By their very nature, scientific codes are built to investigate systems where the behaviour is to some extent unknown, so testing them can be quite difficult. They can be very complex, built over a number of years (or even decades!) with contributions from many people. In this talk, I shall look at how to design tests for scientific codes, showing that even for the most complicated of codes, there are a number of different tests we can apply to ensure we build robust, reliable code.