The support of mobile workers with mobile IT solutions can create dremendous improvements in mobile business processes of a company. The main charateristic of such a mobile system is the ability to connect via a (mobile) network to a central server, e.g. in order to access customer data. The frequency and the location of the use, data topicality, interaction requirements and many more are central aspects when developing a suitable system architecture. This paper provides a detailed decription of the four main software architectures for mobile systems and their main charateristics. Beyond, typical business requirements are developed, the implications for the system architecture for each of them is shown. Keywords. Mobile system, System Architecture, Always-online systems, Business alignment