The main by-products from the oil palm industry are palm kernel expeller (PKE), palm oil sludge (POS), palm pressed fibre (PPF), oil palm fronds (OPF) and oil palm trunks (OPT). The use of these by-products in ruminant feeding systems has been widely reported in Malaysia, and it is estimated that 2.2 million tonnes PKE, 2.2 million tonnes POS, 13 million tonnes of PPF, 35 million tonnes OPF and 9 million tonnes of OPT were available as feeds for ruminants in 2008. This article discusses the utilisation of oil palm by-products as ruminant feed in Malaysia. Studies on the characterisation and digestibility of PKE, PPF, POS, OPT and OPF and their use in practical rations for beef and dairy production have been reported. Factors which can affect nutrient utilisation include the chemical and physical structure of the feeds, feed intake, rate of passage and digestibility of the nutrients. Nutritive value data show that PKE with total digestible nutrients (TDN) of 75.8% and metabolisable energy (ME) of 11.13 MJ kg-1 is a high quality feed while POS (TDN 62.5%, ME 8.37 MJ kg-1) is a feed of moderate quality for ruminants. OPF (ME 5.65 MJ kg-1) and OPT (ME 5.95 MJ kg-1) are lower quality feeds and can be sources of roughage for ruminant livestock. The highly fibrous nature of PPF is reflected by its low degradability in the rumen and its low TDN (27.8%) and low ME (4.21 MJ kg-1) values. Due to its huge quantity and availability throughout the year, OPF has been widely used as a roughage source and as a component in complete rations for ruminant livestock. The optimum level of OPF inclusion in rations for ruminant feeding is 30%. Cattle fed OPF-based diets can achieve weight gains of 600-850 g per day.