A tenderloin is a cut of beef mainly comprised of a muscle known as the psoas major. This long muscle is located behind the kidney in the upper rear section of a cow's loin. It extends from the hip bone and ends just before the ribcage, where the muscle narrows.
A tenderloin is considered the most tender section of beef and is the source of expensive high-end cuts, such as filet mignon. This muscle lacks the toughness of other beef cuts because the psoas major lies in an area of the cow's body that's rarely exerted. The tenderloin is surrounded by kidney fat and flanked by a thin lengthwise chain of muscle, known as the psoas minor, both of which are removed before consumption.
A whole tenderloin cut is often roasted, broiled or grilled to quickly brown the meat while preventing it from drying out. The thicker rear section of tenderloin near the hip bone can be cut into steaks for dishes such as Chateaubriand, while the center section produces the size consistency needed for filet mignon. The thin portion near the ribcage, known as the tail, may be left on a whole roast or cut into smaller pieces for dishes such as kabobs. The front portion of the tenderloin can also be kept intact as part of a larger short loin cut, the thick side portions of the cow between the ribs and hips. This technique is used for popular steak cuts, including Porterhouse and T-bone.