1. A system for visually guiding a flexible endoscope through linked passages within a body, comprising: (a) a signal source that emits a reference signal useful for spatially tracking a progress of the flexible endoscope through the linked passages, as the flexible endoscope is flexibly bent around corners while being advanced through the linked passages; (b) a sensor that produces an output signal indicating coordinates that define a three-dimensional disposition of a distal end of the flexible endoscope using the reference signal of the signal source to continuously determine the three-dimensional disposition of the distal end as it is advanced through the linked passages; (c) a three-dimensional model of the linked passages; (d) a processor for determining positions of branching points and for providing indicators that indicate positions of branching points on the three-dimensional model at which the linked passages connect or diverge, said processor specifying at least initially, at least one predefined path on the three-dimensional model from a plurality of paths formed by the linked passages connecting or diverging, along which to advance the flexible endoscope through the linked passages; and (e) a display for displaying a view from within the linked passages as the flexible endoscope is advanced therethrough, to enable the flexible endoscope to be visually guided and tracked along a path actually followed through the linked passages, the output signal providing a record of the path followed as the flexible endoscope is advanced and enabling the display to show a current position of the endoscope, relative to the three-dimensional model of the linked passages.