[wideawake]

Depends on which army, but a squad is about ten men, a platoon is four squads, a company is two or more platoons, a battalion is two or more companies, a regiment is two or more battalions, a brigade is two or more regiments, a division is two or more brigades, a corps is two or more divisions and an army is two or more corps.

[Tallguy]

a brigade is two or more regiments... Brigades & Regiments are, administratively speaking, equivalent in todays army. Brigades are generally composite in nature, ie. 2 Mech Battalions, plus 1 Tank Battalion.

Today Regiments largely live on paper, except with the Marines, Cavalry and Airborne units which are "purer" formations (think regimented). For example, an Airborne Regiment of the 82nd Abn Div. is almost all light infantry. Armored Cavalry Regiments are composite units that function like mini-divisions.