Introductions are important because readers rely on them to establish the topic and purpose of the document. In technical writing, the introduction often takes a very specific rhetorical structure, which is closely related the structure of engineering papers outlined in Online Handbook / Accurate Documentation / Conducting & Understanding Secondary Research in Engineering.
In order to effectively introduce the reader to a topic, the introduction should:
- Provide context for the report
- Reveal a gap or a problem
- Show how the report fills that niche 
1. Context: Providing context involves establishing why the report is being written and the background information necessary to understand the problem. How much and what type of context we provide in the introduction will depend on the audience. A knowledgeable and in-the-loop audience – such as an immediate supervisor or an engineer who is working on the same project – may already possess the much of the necessary context knowledge: they are known as High Context readers. Low Context readers – such as clients – have less knowledge of the immediate field and situation, and will need you to establish the framework for the paper .
2. Gap: Identifying the gap involves establishing the problem that the report deals with . In many cases, the “problem” is relatively easy to identify. For example, a proposal for a bridge design involves a real, physical gap and the need (to accommodate traffic patterns, for safety reasons, etc.) for a bridge. However, in some genres of engineering writing, such as undergraduate lab reports, no practical problem is actually solved.
When trying to identify a gap, consider the following possibilities:
- Sometimes the gap is not a practical problem, but missing knowledge, such as our inability to understand the mechanisms of Parkinson’s disease.
- In some cases, the “problem” is the need to confirm known knowledge: this is often the case in undergraduate lab reports, where the goal is the validation of scientific theory through experimentation.
- There may also be multiple levels of the problem. For example, osteoporosis is a medical condition that biomedical engineers are trying to fight through different technologies for growing bone. But the real problem that the paper addresses might be the limitations of current technologies, such as calcium supplements, for dealing with osteoporosis, or even bone growth techniques developed by other researchers.
3. Niche: Having identified the gap very precisely, we should have carved out a very specific niche for our paper. By providing 1) a purpose statement and 2) a projection for the paper in the introduction, we identify how we plan to fill that niche.
The purpose statement is establishes what the paper does to fill the niche :
(Problem = confirm known sample composition)
This report explores the feasibility of using synthetic polymers as scaffolding in bone tissue engineering.
(Problem = scaffolding with other materials in BTE)
The projection provides an overview of paper: it explains, by taking us through the steps in logic, how the paper accomplishes its purpose .
This paper first discusses the limitations of using natural polymers in bone tissue engineering applications. We then explain the techniques developed by Smith and Wenner  that address these limitations by combining natural materials with synthetic ones, forming a hybrid material that is both strong and flexible. Finally, we identify the developments necessary for this new material to become a viable scaffolding material for bone tissue engineering.
Two writing issues arise in the above projection passage: the use of the first person and anthropomorphism, attributing human qualities/capabilities to non-human things (such as “the paper ‘discusses'”). While generally acceptable, some readers may prefer that you avoid both. If unsure, ask your primary audience. In any case, the above sentences can easily be rewritten to avoid both issues. (In this paper, the limitations . . . are discussed . . . Second, the techniques . . . are explained.)
This rhetorical structure (context-gap-niche) is specific to engineering writing and depends on readers’ expectations of this particular genre. But introductions to engineering papers still need to follow some of the more general rules and features of good introductions, and will need to manage the following dilemmas:
Length: An introduction should be relatively short; it is, after all, an introduction, and you will have the opportunity to develop the gap further in the literature review or background. However, it still needs to be persuasive – establish the significance of the problem – and provide sufficient and appropriate context information for its readers (audience specific).
Placement of purpose statement: Putting the purpose statement at the beginning means that your audience knows the goal of the report early; but it may also mean that your audience may not understand the need for the work (without sufficient context) .
 Irish, R., Tiede, K., and Weiss, P. Communication Course Notes. Engineering Communication Program, Faculty of Applied Science and Engineering,University of Toronto. 2004.