The purpose of this thesis is to investigate to what extent XML (Extensible Markup Language) based languages are suitable for use as user interface definition languages in terms of their flexibility. The primary method used is qualitative analysis of scientific papers, literary resources on the target field, practical experiences as well as certain essential non-scientific literature such as standards documents and more practically oriented reference literature as well as on-line resources. XML has been shown to support international characters, the separation of user interface and logic as well as cross platform development very well. The derivative standards utilize these fundamental characteristics of XML in this context and add their own contribution to the mix. Our conclusion is that XML as such is ideally suited for creating flexible user interface definition languages. By flexible we mean that they exhibit some degree of all the flexibility factors reusability, customizability and localizability. The p articular XML based languages we have looked at, namely XUL (XML-based User Interface Language) and UIML (User Interface Markup Language) have also been found to add varying degrees of extra weight to the flexibility factors already found in XML. The findings indicate for example that the levels of abstraction in a particular derived language greatly affect the level of additional flexibility achieved through the use of the language in question.