The high incidence of obesity and the lack of safe pharmaceutical agents have fuelled an increase in researches related to anti-obesity drugs. Although a number of pharmacological approaches have been investigated in recent years, few safe and therapeutically effective products have been developed. Phytochemicals are components of plants that convey healthful properties beyond their use as macronutrients or micronutrients. These compounds have biological properties such as antioxidant, modulation of detoxification enzymes, stimulation of the immune system, reduction of platelet aggregation, and modulation of hormone metabolism. Furthermore, the latest discoveries and studies on the cellular and molecular mechanisms of such phytochemicals suggest that they are potential agents in the treatment of obesity and associated diseases, and may be incorporated in food ingredients, dietary supplements, or drug components. The main focus of this chapter is to review the available information on various aspects of phytochemicals, with special reference to their effectiveness in reducing obesity and obesity-related diseases. The bioactives that have been derived from plants, including flavonoids, terpenoids, phenolic acids, and other categories of phytochemicals based on their structure, have shown interesting effects on adipose tissue such as the induction of apoptosis, the inhibition of adipocyte differentiation and lipid accumulation, and the induction of lipolysis. Besides the ample evidence of the anti-obesity effects of these phytochemicals in literature, the characterization of their properties and the accumulation of preclinical data could raise the possibility of a new application of these interesting phytochemicals as novel drug candidates or dietary supplements.