A steerable sheath for use with one of several optical catheters is provided to be positioned in a passageway leading to a body cavity of a patient for observation and/or treatment. The apparatus includes an elongated, deformable, hollow body having a distal end and a proximal end. An optical catheter is removably extendable through the body and has a distal end which is alignable with the distal end of the body and a proximal end extendable outwardly beyond the proximal end of the body. An articulatable hollow, cylindrical member is provided within the sheath body at the distal end thereof and extends along the sheath body for a short distance toward the proximal end. A first generally cylindrical yoke is pivotally mounted about an axis at the proximal end of the sheath body. The proximal ends of a first pair of wires which extend longitudinally within the sheath body from the distal end thereof to the proximal end are attached to the first yoke and offset 90.degree. from the first yoke axis. A first knob is connected to the first yoke for rotating it back and forth about the first yoke axis to alternately push and pull the first pair of wires to steer the cylindrical member in an appropriate direction. A method is also contemplated using the apparatus in conducting an investigation and/or operative procedure.