The nutritional characteristics of rumen-protected protein and fat supplements produced by formaldehyde treatment of oilseeds and meals are reviewed. The proportion of rumen undegraded protein (RUP) in different protein sources can be controlled by this process, bio-available lysine is 82–84% and the proportions of acid detergent and neutral detergent insoluble nitrogen are unchanged by formaldehyde treatment; this is in contrast to heat treatment of proteins where significant increases in these nitrogen components can occur if the RUP content exceeds 60% of the crude protein (CP). A RUP content of 75–80% of CP is optimal when using protein supplements for milk production, and for body growth in steers a lower RUP content is desirable (i.e. 50–55% of CP). Both the fat and protein constituents in rumen-protected fat supplements derived from the emulsification and formaldehyde treatment of oilseeds are highly protected from ruminal metabolism (75–90%) and are readily digested in the small intestine (90% for C₁₈ unsaturated fatty acids, 82% for the essential amino acids). Protected fat/protein supplements are designed and fed to lactating and non-lactating ruminants to increase efficiency of production, enhance product quality, augment n-3, n-6 and n-9 fatty acid content of meat and milk, and to improve reproductive performance. The challenges and potential role for these protected fat/protein supplements in improving productivity and quality of ruminant derived foods are discussed.