This chapter discusses the characteristics of rumen bacteria that have been successfully cultivated in the laboratory. It highlights that only a small fraction (often less than 1%) of the total microbial diversity in the rumen has been recovered through culturing methods, which may lead to misleading descriptions of the ecosystem. Culturable counts vary significantly from direct microscopic counts, and discrepancies may be due to changes in the viability of culturable species. Advances in anaerobic methods and isolation media have improved the cultivation of rumen bacteria, but some functionally important groups, such as obligate syntrophs, may still not have been recovered. Molecular studies are revealing the true extent of bacterial diversity in the rumen, showing that some existing species encompass significant genetic variation. Current species type strains may not adequately represent the culturable variation in the rumen. Laboratory strains may not accurately reflect the rumen ecosystem due to their ability to evolve under laboratory conditions. Standard media with rumen fluid are sometimes called 'habitat-simulating', but they differ from normal rumen conditions by including high concentrations of sugars, yeast extract, and protein hydrolysates. Rumen bacteria can be grown in clarified rumen liquor, and their growth rates reflect their abundance in the rumen. The addition of yeast extract can stimulate the growth of less abundant species. The open nature of the rumen ecosystem leads to the recovery of many bacterial species, some of which may be transients. This contributes to the diversity of the flora but also introduces arbitrariness in choosing bacteria with functional roles. The chapter also reviews the wide range of aerobic, facultative, and anaerobic organisms found in the rumen, especially in young ruminants.This chapter discusses the characteristics of rumen bacteria that have been successfully cultivated in the laboratory. It highlights that only a small fraction (often less than 1%) of the total microbial diversity in the rumen has been recovered through culturing methods, which may lead to misleading descriptions of the ecosystem. Culturable counts vary significantly from direct microscopic counts, and discrepancies may be due to changes in the viability of culturable species. Advances in anaerobic methods and isolation media have improved the cultivation of rumen bacteria, but some functionally important groups, such as obligate syntrophs, may still not have been recovered. Molecular studies are revealing the true extent of bacterial diversity in the rumen, showing that some existing species encompass significant genetic variation. Current species type strains may not adequately represent the culturable variation in the rumen. Laboratory strains may not accurately reflect the rumen ecosystem due to their ability to evolve under laboratory conditions. Standard media with rumen fluid are sometimes called 'habitat-simulating', but they differ from normal rumen conditions by including high concentrations of sugars, yeast extract, and protein hydrolysates. Rumen bacteria can be grown in clarified rumen liquor, and their growth rates reflect their abundance in the rumen. The addition of yeast extract can stimulate the growth of less abundant species. The open nature of the rumen ecosystem leads to the recovery of many bacterial species, some of which may be transients. This contributes to the diversity of the flora but also introduces arbitrariness in choosing bacteria with functional roles. The chapter also reviews the wide range of aerobic, facultative, and anaerobic organisms found in the rumen, especially in young ruminants.