The Chief of Staff of the United States Army (CSA) is the professional head of the United States Army who is responsible for ensuring readiness of the Army. As with the other military service chiefs, he has no operational command authority. However, under 10 U.S.C. § 743, the chief of Staff of the Army is by default the highest ranking officer in the Army (save the chairman or vice chairman of the Joint Chiefs of Staff, should they be Army officers). He is a member of the Joint Chiefs of Staff and is the chief military adviser to the President of the United States on Army matters. Originally, the officer who was appointed chief of Staff did not have to be in senior rank because he did not have to wait to be appointed until the end of his career. Since the Army has capped all officers with a maximum age set by rank, most chiefs of Staff have been four star generals. 10 U.S.C. § 3033(b) requires all chiefs of staff to be a four star general while holding the position. The current occupant of the office is General George Casey.
Prior to 1903, the military head of the Army was the Commanding General of the United States Army.

U.S. Army Chiefs of Staff

LTG Samuel Young (15 Aug 1903-8 Jan 1904)
LTG Adna R. Chaffee (9 Jan 1904-14 Jan 1906)
LTG John C. Bates (15 Jan 1906-13 Apr 1906)
MG J. Franklin Bell (14 Apr 1906-21 Apr 1910)
MG Leonard Wood (22 Apr 1910-21 Apr 1914)
MG William W. Wotherspoon (22 Apr 1914-16 Nov 1914)
MG Hugh L. Scott (17 Nov 1914-22 Sep 1917)
GEN Tasker H. Bliss (23 Sep 1917-19 May 1918)
GEN Peyton C. March (20 May 1918-30 Jun 1921)
Gen of the Armies John J. Pershing (1 Jul 1921-13 Sep 1924)
MG John L. Hines (14 Sep 1924-20 Nov 1926)
GEN Charles P. Summerall (21 Nov 1926-20 Nov 1930)
GEN Douglas MacArthur (21 Nov 1930-1 Oct 1935)
GEN Malin Craig (2 Oct 1935-31 Aug 1939)
GA George C. Marshall (1 Sep 1939-18 Nov 1945)
GA Dwight D. Eisenhower (19 Nov 1945-6 Feb 1948)
GEN Omar N. Bradley (7 Feb 1948-15 Aug 1949)
GEN J. Lawton Collins (16 Aug 1949-14 Aug 1953)
GEN Matthew B. Ridgway (15 Aug 1953-29 Jun 1955)
GEN Maxwell D. Taylor (30 Jun 1955-30 Jun 1959)
GEN Lyman L. Lemnitzer (1 Jul 1959-30 Sep 1960)
GEN George H. Decker (1 Oct 1960-30 Sep 1962)
GEN Earle G. Wheeler (1 Oct 1962-2 Jul 1964)
GEN Harold K. Johnson (3 Jul 1964-2 Jul 1968)
GEN William C. Westmoreland (3 Jul 1968-30 Jun 1972) GEN Bruce Palmer, Jr. (Acting) (1 Jul 1972-11 Oct 1972)
GEN Creighton W. Abrams (12 Oct 1972-4 September 1974)
GEN Frederick C. Weyand (3 October 1974-30 September 1976)
GEN Bernard W. Rogers (1 October 1976-21 June 1979)
GEN Edward C. Meyer (22 June 1979-21 June 1983)
GEN John A. Wickham, Jr. (23 July 1983-23 June 1987)
GEN Carl E. Vuono (23 June 1987-21 June 1991)
GEN Gordon R. Sullivan (21 June 1991-20 June 1995)
GEN Dennis J. Reimer (20 June 1995-21 June 1999)
GEN Eric K. Shinseki (21 June 1999-11 June 2003)
GEN Peter J. Schoomaker (1 Aug 2003-10 Apr 2007)
GEN George W. Casey, Jr. (10 Apr 2007-Present)
Source: Historical Resources Branch, United States Army Center of Military History