Abstract. There are insufficient examples available of how customization is realized in multi-tenant web applications, whereas developers are looking for examples and patterns as inspiration for their own web applications. This paper presents an overview of how variability realization techniques from the software product line world can be applied to realize customization when building multitenant web applications. The paper addresses this issue by providing a catalogue of customization realization techniques, which are illustrated using occurrences of customization in two practical innovative cases from the library sector. The catalogue and its examples assist developers in evaluating and selecting customization realization techniques for their multi-tenant web application.