One challenge for development teams designing medical devices is limited access to the complex clinical use environment for which the devices are destined, which complicates testing the user-product and patient-product interactions in the product development process. The arrival of iterative development methods exacerbates this problem, as testing in short development loops requires the availability of appropriate test environments. The objective of this thesis is to investigate how testing can be effectively integrated into a iterative product development process for medical devices. This thesis focuses on three aspects: The role of functional product testing in iterative product development, the testing of user interaction in the clinical environment, and the integration of patient interaction testing, using the example of infusion therapy. We investigate the role of functional product testing in iterative product development in the first part. Despite the importance and variety of testing activities, it has not been shown whether testing activities contribute to success in agile development of physical products. We conduct an observational study with 355 students grouped into 72 teams to investigate the link between testing activities and success in agile product development. Each team developed a physical product over the course of a 12-week project. The project structure incorporated several aspects of the agile method Scrum. At the end of the project, the performance of each team’s product was evaluated with a performance score. Statistical analysis was carried out using multiple regression models. The results show that time invested early in the project and testing activities throughout the project were statistically significant predictors of success. Furthermore, the results highlight the variety of performed testing activities. The results of the study underline the importance of testing and provide quantitative empirical evidence supporting conjectures discussed in the literature. In the second part, we turn to the testing of the product-user interaction. The user’s gaze can provide important information about human-computer interaction, but the analysis of manual gaze data is extremely time-consuming, which inhibits its wider adoption in usability studies. The second study presents an algorithm for automated Dynamic AOI Mapping (aDAM), which allows the automated mapping of gaze data recorded with mobile eye tracking to the predefined AOIs on tangible screen-based UIs. The evaluation of the algorithm is performed using two medical devices, which represent two extreme examples of tangible screen-based UIs. The algorithm requires some additional initial input for the setup and training, but analysed gaze data duration and effort is only determined by computation time and does not require any additional manual work thereafter. The accuracy and robustness of both the automated gaze mapping and the screen matching indicate that aDAM can be applied to a wide range of products. Due to the efficiency of the approach, there is a potential for the broader adoption of mobile eye tracking in usability testing during product development, and may contribute to a more data-driven usability engineering process in the future. The patient-product interaction for syringe infusion pumps was explored in the third part. The effect of the patient on the pump, changes in venous pressure induced by postural changes, and vertical relative displacements were modelled using a pressure interface on a test-bench to mimic changes in venous pressure and hydrostatic pressure changes dynamically and fully automatically. The effect of flow rate variability on the patient was modelled using pharmacokinetic simulation models for a neonate receiving an adrenalin infusion. The model takes flow measurement data from the infusion pump as input to simulate the blood drug concentration of the patient. We conduct a study based on these simulations to evaluate the performance of seven modern syringe infusion pump assemblies at low infusion rates, as well as their impact on drug concentration. This study demonstrated that problems with the performance of infusion syringe pump assemblies have remained unsolved, while the pharmacokinetic model revealed considerable impacts on plasma drug concentration when highly concentrated short-acting cardio-vascular drugs were administered at low flow rates. The problems are mainly related to the functional principle of syringe infusion pumps, affecting all tested assemblies Finally, based on an analysis of the problems affecting current syringe infusion pumps, a novel, flow-controlled syringe pump system was designed and compared to a standard infusion syringe pump on an in-vitro test bench in a fourth study. The novel pump almost completely eliminates delays at start-up and flow irregularities during hydrostatic pressure changes. Related fluctuations in plasma drug concentration are minimised and the known disadvantages of standard syringe infusion pumps currently used in clinical practice reduced. Overall, the results of the thesis suggest that testing in iterative product development is effective when it is employed throughout the entire project, taking into account user as well as patient interaction, as unexpected interactions with the device can be captured and addressed early on. The efficiency of the methods presented in this work, which explicitly address the user and patient interfaces during product development, may help to integrate them more effectively into the development of future medical devices. This might contribute to the development of safer medical devices in the future.