A crime boss, crime lord, mob boss, kingpin, criminal mastermind, or Don is a person in charge of a criminal organization. A boss typically has absolute or nearly absolute control over his subordinates, is greatly feared by his subordinates for his ruthlessness and willingness to take lives in order to exert his influence, and profits from the criminal endeavours in which his organization engages.
Some groups may only have as little as two ranks (a boss and his soldiers). Other groups have a more complex, structured organization with many ranks, and structure may vary with cultural background. Organized crime enterprises originating in Sicily differ in structure from those in mainland Italy. American groups may be structured differently from their European counterparts, and Latino and African American gangs often have structures that vary from European gangs. The size of the criminal organization is also important, as regional or national gangs have much more complex hierarchies. 
The boss in the Sicilian and American Mafia is the head of the crime family and the top decision maker. A boss will typically put up layers of insulation between himself and his men in order to defeat law enforcement efforts to arrest him. Whenever he issues orders, he does so either to his underboss, consigliere or capos. The orders are then passed down the line to the soldiers. This makes it difficult under most circumstances to directly implicate a boss in a crime, since he almost never directly gives orders to the soldiers. Only the boss, underboss or consigliere can initiate an associate into the family, allowing them to become a made man. The boss can promote or demote family members at will. If the boss is incarcerated or incapacitated he places an acting boss who responsible for running the crime family. When a boss dies the crime family members choose a new boss.
The typical structure within the Mafia in Sicily and America is usually as follows:
- Boss of all bosses – also known as the capo di tutti capi or godfather, has been given by the media to the most powerful boss, although the Mafia never recognized the position itself. The highest body to decide on inter-family issues is the Commission (see also Sicilian Mafia Commission).
- Boss – Also known as the capofamiglia, capo crimini, representante, Don or godfather, is the highest level in a crime family.
- Underboss – Also known as the "capo bastone" in some criminal organizations, this individual is the second-in-command. He is responsible for ensuring that profits from criminal enterprises flow up to the boss, and generally oversees the selection of the caporegime(s) and soldier(s) to carry out murders. The underboss may take control of the crime family after the boss's death. He keeps this position until a new boss is chosen, which in some cases was the Underboss.
- Consigliere – Also known as an advisor or "right-hand man," a consigliere is a counselor to the boss of a crime family. The boss, underboss, and consigliere constitute the "Administration." The consigliere is third ranked in the hierarchy but does not have capos or soldiers working for him. Like the boss, there is usually only one consigliere per criminal organization.
- Caporegime – Also known as a captain, skipper, capo, or "crew chief," the caporegime was originally known as a "capodecina" (captain of ten) because he oversaw only 10 soldiers. In more recent times, the caporegime may oversee as many soldiers as he can efficiently control. A caporegime is appointed by the family boss to run his own borgata (regime, or crew) of soldato (soldiers). Each caporegime reports directly to the underboss, who gives him the permission to perform criminal activities. If the family needs to murder someone, the underboss normally asks a caporegime to carry out the order. The caporegime runs the day-to-day operations of his crew. The caporegime's soldiers give part of their earnings to him, and then he gives a share to the underboss. A caporegime can recommend to the underboss or boss that a recruit be allowed to join his crew as a mob associate.
- Soldato – Also known as a sgarrista, soldier, "button man," "made man", "wiseguy" or "goodfella", is the lowest level of mobster or gangster. A "soldier" must have taken the omertà (oath of silence), and in some organizations must have killed a person in order to be considered "made." A picciotto is a low-level soldier, usually someone who does the day-to-day work of threatening, beating, and intimidating others.
- Associate – Also known as a "giovane d'onore" (man of honor), an associate is a person who is not a soldier in a crime family, but works for them and shares in the execution of and profits from the criminal enterprise. In Italian criminal organizations, "associates" are usually members of the criminal organization who are not of Italian descent.
The term Mr Big is used within the underworld, and within media reportings of persons associated with criminal activities, to refer to a leader of a body of persons functioning in the capacities of roles within organised crime. Sometimes bosses of the so-called gangland are referred to as being Mr Big, as for example in the case of a report of the attempted assassination of an apparent leader of the Irish Republican Army, a person named Alan Ryan.  The term implicitly indicates a degree of a possession of a higher intelligence of an individual. 
The term especially indicates the existence of involvement in what is known as big-time crime, which would include for example armed robbery, and the more organised aspects of careers within crime. 
In the vernacular of underworld lexiconography within the 1940s of the United States of America, one source does not include the term as a known slang term, but does list Big Brains, as referring to a gang leader. 
- Drug lord – bosses of drug cartels
- List of crime bosses
- List of criminal enterprises, gangs and syndicates
- List of fictional crime bosses
- Organized crime
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