A design document is a written outline of the development of a course or a description of a software product that a software designer writes in order to give a software development team an overall guidance of the architecture of the software project.

Instructional designers (used by corporate training entities) create design documents to articulate how a course will be developed. It generally describes the learning objectives, the modules, learning assessment methods, and source content.

When the design document is description of a software product, it usually accompanies an architecture diagram and has pointers to detailed feature specifications of smaller pieces of the design. Practically, a design document is required to coordinate a large team under a single vision.

A design document needs to be a stable reference and outline all parts of the software and how they will work. The document should give a fairly complete description while maintaining a high-level view of the software.

The Software Design Document (SDD) is a comprehensive software design model consisting of four distinct but interrelated activities: data design, architectural design, interface design, and procedural design.

The SDD contains the following documents:

1. Data Design
2. Architecture Design
3. Interface Design
4. Procedural Design

1. The Data Design describes structures that reside within the software. Attributes and relationships between data objects dictate the choice of data structures.

2. The Architecture Design uses information flow characteristics, and maps them into the program structure. Transformation mapping method is applied to exhibit distinct boundaries between incoming and outgoing data. The Data Flow diagrams allocate control input, processing, and output along three separate modules.

3. The Interface Design describes internal and external program interfaces as well as the design of human interface. Internal and external interface design are based on the information obtained from the analysis model.

4. The Procedural Design describes structured programming concepts using graphical, tabular, and textual notations. These design mediums enable the designer to represent procedural detail that facilitates translation to code. This blueprint for implementation forms the basis for all subsequent software engineering work.

There are two kinds of design documents: HLDD - High Level Design Document and LLDD - Low Level Design Document