Through the use of a variety of implantable devices, significant progress towards an understanding of the neuronal basis of behavior is being achieved. Also, there has been an increased interest in the use of implantable devices for the treatment of numerous neurological disorders. Unfortunately, the design of instruments to be inserted into neuronal tissue has largely been one of trial and error and has not utilized approaches to minimize tissue damage. This is largely due to the lack of understanding of the mechanics of probe insertion and the mechanical properties of neuronal tissue at the relevant length scales. Therefore, state of the art probes generally rely on rigid materials that are not suited for the study of small animals, such as mice, or for general treatment of human neurological disorders. The goal of this work is to provide a generalizable description of the micrometer scale penetration mechanics and material properties of mouse brain tissue in vivo. Cylindrical stainless steel probes were inserted into the cerebral cortex and olfactory bulb of mice under anesthesia. The effects of probe size, probe geometry, insertion rate, insertion location, animal age and the presence of the dura and pia on the resulting forces were measured continuously throughout probe insertion and removal. Material properties (modulus, cutting force, and frictional force) were extracted using mechanical analysis. Implications for future probe design strategies and insertion methodologies will be presented.