The thesis statement of an essay identifies what the essay is about and why the subject of the essay is being discussed--its Significance, Importance, Relevance, or Value (SIRV). While the thesis may appear elsewhere in the essay, most readers (and teachers) expect the thesis to appear as the last sentence of the introduction. Sometimes students will begin an essay with just a thesis statement. The problem in doing this is that the thesis statement usually requires some background information be presented to help the reader understand the thesis. Some instructors may require students to underline the thesis statements in their essays, but the thesis statement should be easily recognizable by its role in stating the main idea of the essay, whether or not it is underlined.

In most cases, the topic of an essay is the subject of the thesis sentence and the focus on that topic is clarified by the verb and complement of the thesis sentence. The verb and its complement typically explain the Significance, Importance, Relevance, or Value (SIRV). of the topic under discussion.

Sometimes an essay may have an implied thesis where the purpose of the essay is so clear that it does not need to be stated. However, implied thesis statements are rarely used in most analytical or argument essays. An exception occurs in satire, where the thesis is implied by the ridicule of its opposite, the stated thesis of the essay. So, the implied thesis statement is an ironic criticism of the supposed--and expressed--thesis.

For instance, the thesis of Jonathan Swift's "A Modest Proposal for Preventing the Children of Poor People in Ireland from Being a Burden to Their Parents or Country, and for Making Them Beneficial to the Public" is that poor Irish parents should raise as many children as possible and sell them for food. Swift isn't actually calling for parents to raise their children for food. Instead, his satirical treatment of the subject criticises the flawed and uncaring attitude of his society toward Ireland's poor.