Apparatus and methods are provided for accessing body lumens and/or for delivering instruments into body lumens, e.g., vessels within a patient's vasculature. A flexible sheath (20) is provided that is expandable from a contracted condition to an enlarged condition wherein the sheath at least partially defines a lumen therein. The sheath is lubricious and has a relatively thin wall, thereby providing a collapsible/expandable guide for delivering fluids and/or instruments through tortuous anatomy and/or into relatively narrow passages. The sheath is advanced from an entry site to a body lumen in the contracted condition. Once the sheath reaches a target body lumen, the sheath is expanded to the enlarged condition, thereby defining a lumen within the sheath, and fluids and/or instruments are introduced into the body lumen via the sheath lumen. Upon completing the procedure, the sheath is removed from the body lumen.
