User interaction with online geospatial systems is an inter-disciplinary research field that combines the two disciplines Human Computer Interaction (HCI) and geographic information sciences. In recent years online geospatial systems have been created for different purposes such as way-finding systems, online atlases and collaborative systems. In order to accommodate the fact that an increasing number of people have access to geospatial data and technology, research has begun to focus on the impact and the utilization of such systems. However few studies regarding the usability of online geospatial systems have been conducted and most of these studies were set up in artificial lab-environments with few mostly poorly described test-users. Furthermore to the best of our knowledge no formal research framework currently exists that allows for analyzing the interaction of real-world users with such systems and that takes into account the spatial nature of these systems. The goal of this thesis is to show that users, depending on their demographics, experience, skills and context show significant differences in their interaction strategies and performance. Another important factor that has been neglected in many recent laboratory-based usability studies is the user's computer. In this work, we want to show that users, depending on their computer's properties show significant differences regarding their interaction strategies, their performance and satisfaction. The work in this thesis consists of three parts: 1. the establishment of an evaluation framework that categorizes and defines the interaction of real-world users with geospatial systems. The four main entities of this framework are the user (in terms of demographics, skills, knowledge and context), the system (defined by the interface design, spatial data, spatial functions, technologies and architectures), the user interaction (interaction strategies, performance and perception of spatial data), and the satisfaction of using the system. 2. the development of methods and software tools for gathering experimental data based on the four entities of the evaluation framework 3. the validation of the evaluation framework with case studies involving systems that have been developed for specific groups of end-users. We have conducted three case studies where real-world users interact with three online geospatial systems. The important findings of those studies were that there are noticeable gender differences regarding spatial navigation, that user satisfaction appears to be biased by the user's previous experience with geographic information technologies and that the type of the input device used (e.g. mouse or touchpad) has a considerable influence on task-completion time and user satisfaction. The above results indicate that the user's demographic parameters and background have a significant effect on the user's spatial interaction strategies, performance and satisfaction. Furthermore we have found evidence that suggests that the user's perception of spatial data strongly depends on the scale. We conclude that the design and development of spatial interaction functions (spatial navigation, digitization, selection of objects and locations) in online geospatial systems should highly depend on the type of users that the systems are developed for. To further fuel research on usability aspects of online geospatial systems, we provide the necessary tools and methods for conducting remote- and laboratory based testing of such systems.