Fillet steak is a steak cut of beef taken from the smaller end of the tenderloin. The tenderloin is the most tender cut of beef and is also arguably the most desirable and therefore the most expensive.
Because the muscle is not weight-bearing, it contains less connective tissue, which makes it tender. However, because it is so lean, it is generally not as flavorful as some other cuts of beef. The tenderloin is a long thin muscle, the end of which is known as Filet Mignon. Take care when cooking this cut; do not overcook. Overcooked fillet can be very as fillets have low levels of marbling, or intramuscular fat.