The invention relates to the guidance, positioning and placement confirmation of intravascular devices, such as catheters, stylets, guidewires and other flexible elongate bodies that are typically inserted percutaneously into the venous or arterial vasculature. An aspect of the invention includes, for example, an endovenous access and guidance system. The system comprises: an elongate flexible member adapted and configured to access the venous vasculature of a patient; a sensor disposed at a distal end of the elongate flexible member and configured to provide in vivo non-image based ultrasound information of the venous vasculature of the patient; a processor configured to receive and process in vivo non-image based ultrasound information of the venous vasculature of the patient provided by the sensor and to provide position information regarding the position of the distal end of the elongate flexible member within the venous vasculature of the patient; and an output device adapted to output the position information from the processor.