An overtube for use with an endoscopic surgical instrument. In various embodiments, the overtube may comprise a substantially flexible helically wound continuous member forming a series of helical coils that define a hollow passage sized to receive a portion of the endoscopic surgical instrument therethrough. The coils may be configured to selectively interlock with each other to stiffen the overtube. An actuation system may be employed to steer the overtube and selectively stiffen it. Some embodiments include a second substantially flexible helically wound member that may be selectively wound between the first substantially flexible helically wound member or segments thereof.