Abstract
There is a renewed interest in the study of the possible precession of the central sources of planetary nebulae. Three different morphological features are considered the effect of precession: 1.Point-symmetric PNe: Objects whose morphological components are symmetrical with respect to reflection through the center.2.Quadrupolar PNe: Characterized by two well-defined pairs of lobes which are symmetric, in pairs, with respect to two different axes. These PNe are very likely the result of two episodic nebular ejection processes, produced at different positions of the symmetry axis.3.Bipolar, rotating episodic jets (or BRET's): Spectacular structures (Lopez, Meaburn, & Palmer 1994) due to the precession of a symmetrical, collimated episodic outflow.
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