Autophagy is a process in which cytoplasmic components are degraded in the vacuole. It occurs when organisms are subjected to environmental stress conditions or during certain stages of development. Upon induction of autophagy, a portion of cytoplasm is surrounded by a double membrane structure to form an autophagosome. The outer membrane of the autophagosome then fuses with the vacuole and the inner membrane and its contents are degraded by vacuolar hydrolases. The morphological characteristics of autophagy have been observed in plants for decades and several autophagy defective mutants have been isolated recently; however, little is known about the molecular mechanism and physiological role of the autophagy pathway in plants. This thesis summarizes my efforts in studying the function of the autophagic machinery in Arabidopsis thaliana;To study when and where autophagy is induced in plants, two autophagy specific markers, monodansylcadaverine (MDC) and a green fluorescent protein (GFP)-AtATG8e fusion protein, were developed. To obtain more knowledge on the molecular mechanism of autophagy, an autophagy-related gene AtATG18a was identified by its sequence similarity with the yeast autophagy gene ATG18. RNA interference (RNAi)- AtATG18a transgenic plants with decreased expression of AtATG18a cannot produce autophagosomes in conditions that normally induce autophagy, suggesting that AtATG18a is required for autophagosome formation. Using these two autophagy markers and RNAi- AtATG18a transgenic plants, the physiological roles of autophagy were investigated. Autaphagy was found to play an important role in the control of the timing of senescence and in the response to several abiotic stresses, such as nutrient deprivation, oxidative stress, high salinity stress and osmotic stress. In nutrient deprivation stress, autophagy is involved in recycling unnecessary cytoplasmic components to provide raw materials and energy for survival. In oxidative stress, at least one physiological role of autophagy is to degrade oxidized proteins. Finally, autophagy was found to be regulated independently upon different abiotic stresses. In nutrient deprivation and salt stress, autophagy is regulated by an NADPH oxidase-dependent pathway, whereas in osmotic stress, autophagy is regulated by an NADPH oxidase-independent pathway.