Assistant Commandant is a title often given to the second-in-command of a military, uniformed service, training establishment or academy. This usage is common in English-speaking nations,^{[citation needed]} and in some countries it may be a military or police rank.

Assistant commandant as a rank was envisaged by General J. N. Chaudhuri, who was part of the committee studying the need for a dedicated Border Guarding force raised on military lines to guard the perilous India-Pakistan border given the UN conventions deriding and preventing military formations or build-up on the borders between the two un-friendly neighbors. General J. N. Chaudhuri recommended to raise the officer cadre of the Border Security Force on lines of the military hierarchy and hence assistant commandant, which otherwise is an appointment in most countries, came up as the junior-most rank in the hierarchy of Gazetted officers of the Border Security Force.

For its exclusivity and unique hierarchical association with Indian Paramilitary Forces of India, assistant commandant, for comparative understanding, is equivalent to the ranks of assistant commissioner of police (ACP), deputy superintendent of police (DSP), and captain in the Indian Army. This rank is the highest entry level rank in the Central Armed Police Forces of India.

Assistant commandants in Central Armed Police Forces (CAPF) are selected through a competitive exam conducted by Union Public Service Commission (UPSC) annually.

Assistant commandants are mandated to command companies and Direct Action Teams (DATs) in various CAPFs and law enforcement agencies, They are also sent on deputations to various organizations such as RAW, NSG, SPG, NIA, IB, CBI, NDRF, NCB, State Armed Police Force (DRG, Jharkhand Jaguars, Bihar military police, IRB, UP/MP STFs, etc.). Their role and performance therefore assumes an enormous significance because of the multitudinous role they play at various leadership and staff roles.

Sub-inspectors are recruited through competitive examination conducted by Staff Selection Commission and they are referred to as DASOs (directly appointed subordinate officers). DESOs (departmental entry subordinate officers) are those officers who have been promoted through departmental exams conducted internally for inspectors, sub-inspectors, assistant sub-inspectors, head constables, constables etc.

After joining service as assistant commandant the following promotional avenues are available to cadre officers:-

In the United States, assistant commandant is an appointment, not a rank. A common use of the term is for the position of assistant commandant of the Marine Corps .Appointments to this position are held by officers with the rank of general. Officers who hold this appointment are the second highest ranking officers in the United States Marine Corps after the commandant of the Marine Corps^[1]