In this paper we introduce the topic input validation, analyze its great importance to Web applications and suggest a new comprehensive approach to input validation. The approach has been developed as a result of an evaluation of current input validation approaches that showed that no sufficient solution to common input validation requirements is available at present. The paper describes important requirements for input validation frameworks, especially in the Web context, and introduces main concepts of this approach. The approach is based on the declarative, rule based definition of validation logic and the automatic translation of validation rules into server side and client side code. It supports conditional, composite and complex inter-field validation scenarios. It considers topics such as value normalization, inter-field dependencies and validation actions and integrates these aspects into a consistent validator based system. Our evaluation shows the benefits of this new approach and highlights its advantages compared to other popular and promising approaches such as PowerForms or Topes.