Mobile applications are becoming increasingly widespread and complex. Many of these applications suffer from usability issues, including information overload, screen clutter, lack of task support and limited interaction me chanisms. Adaptive user interfaces (AUIs) have been proposed to address some of these usability issues. The aim of this paper is to investigate how AUIs can improve the usability of mobile applications. This paper discusses several simple types of adaptation that have been shown to yield significant usability benefits for mobile applications. Two case studies are presented to illustrate how an AUI can be incorporated into different types of mobile applications. This paper also discusses the lessons learned from these case studies and presents some implications for designing adaptive systems in the future.