The prevalence of obesity is increasing at an alarming rate worldwide and places a major economic burden on health-care systems. Obesity occurs when energy uptake is greater than energy expenditure, resulting in an accumulation of excess body fat with subsequent damaging physiological changes; this co-occurrence is often referred to as metabolic syndrome. The chapter reviews the proposed mechanisms of action of phytochemicals on obesity-related pathways and identifies commercial Australian native plant sources that possess elevated levels of phytochemicals that are capable of modulating the adipocyte life cycle and/or reduce inflammatory responses. Flavanones, a class of flavonoids found in high concentrations in citrus fruit, have shown promising evidence in preventing weight gain and other components of metabolic syndrome. Scientific investigations using cell culture and animal model studies demonstrate that phytochemicals derived from plant tissues can induce lipolysis, decrease lipid accumulation and induce apoptosis in adipose tissue.