Medical interventions in the central nervous system (CNS) are challenging due to the complexity and delicacy of the brain tissue. Techniques that do not require opening the skull would alleviate patient discomfort and increase post-operative outputs. Vessels are designed to reach every tissue in the human body. Therefore, the exploitation of these hollow conduits represents a unique opportunity to delivery therapeutic and diagnostic tools. With the advances in real-time X-ray imaging technologies and endovascular catheters, minimally invasive cerebral operations have become standard in clinical practice. A catheter is a long and flexible tube that can be navigated inside the blood vessels to perform a variety of different tasks such as deployment of stents and injection of embolizing agents or chemotherapeutics. Catheter-based endovascular interventions are routinely performed in the main and middle vessels. However, navigating catheters inside sub-mm arteries is an open challenge. Since conventional catheters are inserted by pushing, they require sufficient mechanical resistance to avoid buckling, but at the same time, they must guarantee adequate flexibility to conform with the vessel tortuosity. This trade-off has strong repercussions on the manoeuvrability, since an increase in flexibility results in a decrease in pushability and torquability. Accessing microscale vessels requires devices with sections of only tens of microns. At this size scale, even the hardest material cannot provide sufficient mechanical stability against compression. Moreover, such devices raise significant safety concerns due to the increased risks of vessel perforation. This thesis introduces a novel strategy in endovascular navigation. The fundamental difference between the state-of-the-art and the presented technique is the abandonment of the insertion based on proximal push. Ultraflexibility that naturally arises from extreme miniaturization is leveraged to harness the ubiquitous hydrodynamic pull provided by the blood flow. This flow-driven navigation is capable of transporting endovascular instruments to the microvasculature. The adoption of ribbon geometry unlocks the potential of wafer technology, with which electronic sensors are integrated with mechanical structures. The thesis explains the principles of flow-driven navigation in detail using numerical simulations and experimental characterization. We introduce a steering technique based on the application of magnetic torque to navigate at bifurcations. A second navigation technique based on the undulations of magnetic structures is introduced to compensate the lack of propulsive force in vessels with impaired flow conditions. The devices and navigation techniques are validated in ex vivo animal organs. Various measures and techniques are presented that would make a smooth transition to pre-clinical testing.