A deflectable tip introducer or guide catheter optimized for placement of leads or catheters within the vasculature of the human body is disclosed. The introducer includes inner and outer concentrically located tubular sheaths. The inner and outer sheaths are preferably provided with different curved configurations, allowing the over-all displayed curvature of the introducer over its distal portion to be adjusted by longitudinal movement of the inner and outer sheaths relative to one another. The outer sheath may be fabricated of a thin tube of polyetheretherketone (PEEK) or other material having a relatively large stiffness as compared to the inner sheath. The outer sheath is sized to fit closely around the inner sheath. The inner sheath may be formed of a reinforced polyurethane, and preferably has a thin wall that is of about .005 inches in thickness or less. When supported by the inner sheath, the outer sheath transfers torque along the composite introducer assembly and serves to modify the curved configuration displayed by the distal end of the introducer assembly in conjunction with the relative longitudinal movement of the inner and outer sheaths.