As web developers, the applications you write are complex data processing engines. They try and convince your users to enter good, meaningful data and to respond in solid, predictable ways based on what was entered. Robust data validation will allow the rest of you application to work effectively. [...] Data is scrutinized in various ways to make sure that it adheres to basic restrictions and to fundamental properties. It's no good receiving a sandwich when you expect a car.

The questions he answers are:

• Why is Data Validation Important?
• Where Should I be Validating Data?
• When Should My Data Be Validated?
• Who is Responsible for Validation?
• How Do I Validate My Data? (yes, there's a "w" in there!)

In the real world data is ugly, crazy, and untrustworthy. Your only hope to taming the data beast is to diligently, methodically validate your data.