What's more, additionally, furthermore, moreover, and besides are all synonyms for this phrase. These words indicate that there is something else that should be added to the topic that has just been discussed. They are useful for providing more detail or further evidence to support an argument or point of view. What's more and moreover are often used interchangeably. Furthermore is similar to these words but can be slightly more formal. Besides can be used when discussing something that is unrelated or supplementary to the main topic. Overall, these synonyms offer a range of options for adding emphasis and supporting statements in both written and spoken communication.