Medical needles may not be very popular among patients, but they are exceptionally versatile instruments that have found their way into virtually every clinical intervention imaginable. However, despite their versatility, needles can be very difficult to use, and there is much room for improvement. Improvements can be realized by optimization of needle geometry, by the development of training facilities for clinicians, imaging modalities and needle-based sensors, by development of needle-steering mechanisms and path planning methods, or even by fully automating the needle insertion process. These approaches all rely on a proper understanding of the mechanical interaction between needle and soft-tissue. Ideally, insight into needle-tissue interaction mechanics should follow from the development and refinement of theoretical models based on experimental observation. The development of theoretical models has received a lot of attention in the literature, but our inability to collect useful and reliable experimental data remains an important obstacle. For this reason, the present thesis deals with the experimental study of needle-tissue interaction. The goal of the thesis is to provide insight into needle-tissue interaction mechanics based on experimental observation. To achieve this goal we measure the axial component of the external force acting on the needle during interaction with tissue, and we observe the position of the needle relative to the surrounding tissue. The first part of the thesis provides a basis for experimentation. This includes a survey of literature related to needle-tissue interaction force measurements. The intention of this survey is to gather existing experimental evidence regarding the influence of different factors, such as needle type, tissue type, and insertion speed, on the axial force. Based on this survey, a data model is constructed that describes the interrelations between the different aspects of needle-tissue interaction experiments. This data model enables the detailed encoding of experimental equipment, conditions, design, and results, and can be used as the blueprint for a database for experimental needle-tissue interaction data. The second part of the thesis presents two examples of relatively well-controlled experiments involving artificial specimens. These examples illustrate what happens at the tip of a needle during puncture of a membrane. The first experiment involves the use of high speed video to investigate the relation between axial force and needle tip geometry. The second experiment presents a follow-up study that investigates the influence of needle coating (lubrication) on the axial force during membrane puncture. The last part of the thesis presents two examples of experiments involving biological tissue. The first example is an exploratory study aimed at the characterization of forces during needle insertion into the kidney of a human cadaver. This is done with the help of synchronized ultrasound visualization. The second example is concerned with needle insertion into isolated porcine kidneys, intended to achieve a more detailed characterization of forces for different anatomical structures inside the kidney. This experiment uses synchronized ultrasound visualization with the ultrasound probe moving along with the needle. The thesis contributes to the field of needle-tissue interaction mechanics by providing an overview of available knowledge concerning needle-tissue interaction forces, and by providing a framework for structuring and expanding this knowledge. The thesis provides some insight by illustrating how needle reuse, needle coating, and specimen boundary conditions influence specific force metrics in artificial specimens. In addition, the thesis provides a first impression of the variability encountered during needle insertion into human cadaverous kidneys, and it provides a multivariate stochastic model of membrane puncture forces in porcine kidneys that can be used to simulate puncture events. However, the most important contribution consists of a set of tools for gathering, analyzing, and disseminating experimental needle-tissue interaction data.