RevMexAA (Serie de Conferencias), 9, 201-209 (2000)

Our understanding of the formation of planetary nebulae (PNe) has been profoundly inuenced in recent years by the detection of high-velocity, collimated outows in these objects. Outows reaching expansion velocities of several hundred km s and evidences of episodic, multiple ejection events since the proto-PNe stage, have radically modi ed our previous concepts of the evolution of post-AGB objects. Hydrodynamic models alone seem now incapable of reproducing many of the attendant dynamical phenomena observed in PNe. MHD models and binary nuclei are thus playing an increasingly important role in interpreting the diversity of collimated outows in PNe. This paper presents an overview of these phenomena and discusses some of the current challenges and trends in the eld