Feeding high-grain diets is a common practice in intensive ruminant production systems to meet animal energy requirements and maximize feed efficiency. One of the consequences of these diets is trans (t)10 shifted ruminal biohydrogenation, which includes increased formation of t10-18:1 and t10, cis (c)12-conjugated linoleic acid (t10, c12-CLA) at the expense of t11-18:1 and c9, t11-CLA. In dairy cows, the t10 shift has been associated with milk fat depression. In beef cattle, the t10 shift has not been associated with negative effects on animal performance or marbling fat deposition; however, it may negatively impact meat fat eating quality by decreasing its cis monounsaturated fatty acid to saturated fatty acid ratio. Finally, the t10 shift increases t10-18:1 accumulation in ruminant-derived foods (dairy, beef, and lamb), which at high enough levels may compromise the nutritional quality of these foods. The present review will cover our current understanding of the rumen microbes involved in the t10 shift and its implications for milk fat synthesis and beef fat quality.