Technological Thoughts by Jerome Kehrli

Test Driven Development is a development practice from eXtreme Programming which combines test-first development where you write a test before you write just enough production code to fulfill that test and refactoring.
TDD aims to improve the productivity and quality of software development. It consists in jointly building the software and its suite of non-regression tests.

The principle of TDD is as follows:

1. write a failing test,
2. write code for the test to work,
3. refactor the written code,

and start all over again.

Instead of writing functional code first and then the testing code afterwards (if one writes it at all), one instead writes the test code before the functional code.
In addition, one does so in tiny small steps - write one single test and a small bit of corresponding functional code at a time. A programmer taking a TDD approach shall refuse to write a new function until there is first a test that fails - or even doesn't compile - because that function isn't present. In fact, one shall refuse to add even a single line of code until a test exists for it. Once the test is in place one then does the work required to ensure that the test suite now passes (the new code may break several existing tests as well as the new one).
This sounds simple in principle, but when one is first learning to take a TDD approach, it does definitely require great discipline because it's easy to "slip" and write functional code without first writing or extending a new test.

In theory, the method requires the involvement of two different developers, one writing the tests, then other one writing the code. This avoids subjectivity issues. Kent Beck has more than a lot of examples of why and how TDD and pair programming fit eXtremely well together.
Now in practice, most of the time one single developer tends to write tests and the corresponding code all alone by himself which enforces the integrity of a new functionalities in a largely collaborative project.

There are multiple perspective in considering what is actually TDD.
For some it's about specification and not validation. In other words, it's one way to think through the requirements or design before one writes the functional code (implying that TDD is both an important agile requirements and an agile design technique). These considers that TDD is first and foremost a design technique.
Another view is that TDD is a programming technique streamlining the development process.
TDD is sometimes perceived as a way to improve quality of software deliverables, sometimes as a way to achieve better design and sometimes many other things.

I myself believe that TDD is all of this but most importantly a way to significantly reduce the "Total Cost of Ownership (TCO)" of software development projects, especially when long-term maintenance and evolution is to be considered.
The Total Cost of Ownership (TCO) of enterprise software is the sum of all direct and indirect costs incurred by that software, where the development, for in-house developped software, is obviously the biggest contributor. Understanding and forecasting the TCO and is a critical part of the Return on Investment (ROI) calculation.

This article is an in depth presentation of my views on TDD and an attempt to illustrate my perspective on why TDD is first and foremost a way to get control back on large Software Development Projects and significantly reduce their TCO.