Criminalization or criminalisation, in criminology, is "the process by which behaviors and individuals are transformed into crime and criminals". Previously legal acts may be transformed into crimes by legislation or judicial decision. However, there is usually a formal presumption in the rules of statutory interpretation against the retrospective application of laws, and only the use of express words by the legislature may rebut this presumption. The power of judges to make new law and retrospectively criminalise behaviour is also discouraged. In a less overt way, where laws have not been strictly enforced, the acts prohibited by those laws may also undergo de facto criminalization through more effective or committed legal enforcement. The process of criminalization takes place through societal institutions including schools, the family, and the criminal justice system.

There has been some uncertainty as to the nature and extent of the contribution to be made by the victims of crime. But, as Garkawe (2001) indicates, the relationship between victimology and criminology has become problematic. The concern is that, within the dialectic of Right Realism and Left Realism, a focus on the victim promotes rights selectively for certain victims, and advocates the assumption that some victim rights are more important than competing rights or values in society. For example, an Islamic feminist might seek consistency of treatment for women as victims and, therefore, demand the decriminalization of abortion, adultery, and seduction (Zina is a Hudud offense in sharia law), and the criminalization of domestic violence and sexual harassment.

In formal academically published theory, the real ruling class of a society reaches a temporary view on whether certain acts or behavior are harmful or criminal. Historically this one theory will be modified by scientific, medical evidence, by political change, and the criminal justice system may or may not treat those matters as crimes.

Conversely, when local politics determines that it is no longer a crime, they may be decriminalized. For example, Recommendation No. R (95) 12 adopted by the Committee of Ministers of the Council of Europe on the management of criminal justice, advocates that crime policies such as decriminalization, depenalisation or diversion, and mediation should be adopted wherever possible. But the law and order debate between right and left politicians is often superficial and unscientific, formulating policies based on their appeal to an uninformed electorate rather than properly conducted research.

Several principles may underpin decisions about criminalization. These include the de minimis principle, that of the minimum criminalization. Under this principle, the general harm principle fails to consider the possibility of other sanctions and the effectiveness of criminalization as a chosen option. Those other sanctions include civil courts, laws of tort and regulation. Having criminal remedies in place is seen as a "last resort" since such actions often infringe personal liberties – incarceration, for example, prevents the freedom of movement. In this sense, law making that places a greater emphasis on human rights. Most crimes of direct actions (murder, rape, assault, for example) are generally not affected by such a stance, but it does require greater justification in less clear cases.

The policy of "social defense" can be seen as an opposing view. It argues that criminalization is used against "any form of activity which threatens good order or is thought reprehensible". The minimization principle may unwittingly prevent the adaptation of the law to new situations. In general, worldwide policy makers have created a myriad of smaller offences, in contradiction to the minimization principle and more in keeping with the social defence.

Leading criminal law philosophers, such as Dennis Baker and Joel Feinberg have argued that conduct should only be criminalized when it is fair to do so. In particular, such theorists assert that objective reasons are needed to demonstrate that it is fair to criminalize conduct in any given case. The commonly cited objective justification for invoking the criminal law is harmful to others, but it cannot deal with all situations. For example, people are not necessarily harmed by public nudity. Feinberg suggests that offence to others also provides an objective reason for invoking the criminal law, but it clearly does not as offence is determined according to conventional morality. Prostitution is another grey area, as some countries allow it in different forms, and it is hard to say whether or not it specifically harms the public in general. One argument may be that prostitution perpetuates the spread of sexually transmitted diseases, including HIV/AIDS, and therefore harms the public which partakes in the act of prostitution. However, the legalization of prostitution would change the way it is regulated, and law enforcement could find a way to prevent the spread of sexually transmitted disease, thus eliminating the health issue and the question of the morality of the profession would be weakened.

People experience a range of physical and social injuries in different contexts which will vary according to the level of economic and political development of their country. Some will be injured out of poverty and malnutrition, others by violence which might stem from a major conflict such as war or from the personal violence in a robbery. The environment may be damaged by pollution, there may be hazards at work. Many of these sources of injury will be ignored while the state may delegate powers of control to a number of different agencies within an international framework where supranational agencies and human rights organisations may offer assistance in responding to the causes of those injuries.