We aimed to update knowledge on relationship between plasma leptin (PL), feed intake (FI) and body fat reserves (BF) in ruminants. A literature review was done, using papers published (n = 31; 1998-2008) in international scientific journals. Studies were done under controlled conditions in dairy (3) or beef (9), goats (1) or sheep (22). Database contained gender, physiological status, breed, age, feeding level and body fatness. Underfeeding decreased PL, basically in fat individuals, while refeeding increased it. The PL depended on physiological status, also being higher in fat vs. leans whereas it decreased during nutritional restriction, following BCS. PL was related to adipose cell size and was modulated by short-term energy intake (EI) in interaction with long-term regulations (i.e. nutritional history). Rather than type of energy, amount of EI and BF are regulators of PL. The PL was positively related to BW changes, feeding level, plasma glucose, β-OH-butyrate, IGF-I and insulin, and negatively related to plasma NEFA. During nutritional restriction, insulin did not correlate with PL. Metabolic effects of PL are thought to be mediated via neuronal systems that possess PL receptors rather than via peripheral effects. There is evidence for a dissociation of PL effects on appetite and neuroendocrine function. Hence, FI and BW increased under long photoperiod and were associated with increases in PL, which was also modulated by daylength. The last have a physiological significance for body fat deposition, which naturally occurs during long days when feed is abundant. The breed had a small effect although PL differences were probably related to breed differences in body composition. Carcass fat can be estimated using PL with the same accuracy than using ultrasound fat thickness. There are positive phenotypic correlations between PL and ultrasound backfat thickness and marbling score, fat depth between 12-13 rib, kidney, pelvic, and heart fat, lean meat yield, and yield grade. Performance (ADG, DMI, and gain:feed) and PL were poorly correlated. There are opportunities to use PL information for optimizing ruminant feeding efficiency and carcass quality.