Crimes are against humanity. Proof of a crime requires proof of some act. Scholars label this the requirement of an actus reus or guilty act. While crimes are typically broken into degrees or classes to punish appropriately, all offences can be divided into 'mala in se' and 'mala prohibita' laws. Both are Latin legal terms. Mala in se meaning crimes that are thought to be inherently evil or morally wrong, and will be widely regarded as crimes regardless of jurisdiction. Mala in se offences are felonies, property crimes, immoral acts and corrupt acts by public officials. Mala prohibita refers to offences that do not have wrongfulness associated with them. Parking in a restricted area, driving the wrong way down a one-way street, jaywalking or unlicensed fishing are examples of acts that are prohibited by statute, but without which are not considered wrong. Mala prohibita statutes are usually imposed strictly as there does not need to be mens rea component for punishment under those offenses, just the act itself.

Criminal liability means responsibility for any illegal behaviour that causes harm or damage to someone or something: The criminal law generally prohibits undesirable acts.

Murder and Culpable homicide: A murder is the act most frequently targeted by the criminal law. In many jurisdictions, the crime of murder is divided into various gradations of severity, e.g., murder in the first degree, based on intent. Malice is a required element of murder. Culpable Homicide is a lesser variety of killing committed in the absence of malice, brought about by reasonable provocation or diminished capacity.

Assault, Battery (crime), Rape, and Sexual abuse: Many criminal codes protect the physical integrity of the body. The crime of battery is traditionally understood as an unlawful touching, although this does not include everyday knocks and jolts to which people silently consent as the result of presence in a crowd. Creating a fear of imminent battery is an assault, and may give rise to criminal liability. Non-consensual intercourse, or rape, is a particularly egregious form of battery.

Property often is protected by the criminal law: Trespass is unlawful entry onto the real property of another. Many criminal codes provide penalties for conversion, embezzlement, theft, all of which involve deprivations of the value of the property. Robbery is a theft by force. Fraud is a breach of the by false representation, by failure to disclose information or by abuse of position.

Example: A person own a liquor store and is diligent about checking ID before selling alcohol to individuals who look younger than 21. But a customer comes in who looks old enough. He checks his ID, which confirms that he is of legal drinking age. Unfortunately, it's a very good fake ID and a police officer who happens to be in the store sees him selling the customer the alcohol. While he had no intention of selling alcohol to an underage person, he may still be charged if his state classifies the selling of alcohol to a minor as a strict liability crime.

If and when one hears the term “strict liability” it's usually in the context of civil cases. More specifically, strict liability comes into play in product liability cases or cases involving animal bites. In civil cases, strict liability claims mean that the plaintiff doesn't have to prove that the defendant acted in a careless manner. For example, if a jurisdiction follows a strict liability theory for dog bites, the dog owner will be financially liable for dog bite injuries regardless of how careful they were with their dog.

Crimes can be divided into a variety of categories, based on the mental state required of the offender. More specifically, crimes can be divided into three categories:

1. General intent: These crimes require that a person intended to perform a particular act, as opposed to intending a certain result. A DUI, for example, is a general intent crime because a driver doesn't have to intend to drive drunk, they just have to be under the influence while driving – but they had the intent to drink alcohol before doing so.
2. Specific intent: These crimes require that a person had the specific intent of committing the particular criminal acts that constitute a crime. A prime example of a specific intent crime is first-degree murder.
3. Strict liability: These crimes don't require any intent, or often knowledge, on the part of the offender.

Strict liability can be described as criminal or civil liability notwithstanding the lack of mens rea or intent by the accused. The category that a crime falls into is important because it affects the elements that must be established to convict a person of committing a particular crime. The prosecutor must prove that the accused fulfilled all elements of the crime, including the required mental state. Not all crimes require specific intent. For example, it might be sufficient to show that an accused acted negligently, rather than intentionally or recklessly. In offences of absolute liability, other than the prohibited act, it may not be necessary to show the act was intentional. Generally, crimes must include an intentional act, and “intent” is an element that must be proved in order to find a crime occurred. The idea of a “strict liability crime” is an oxymoron. The few exceptions are not truly crimes at all – but are administrative regulations and civil penalties created by statute, such as crimes against the traffic or highway code.

1. The most common example of a strict liability crime is statutory rape. This type of rape occurs when a person engages in sexual activity with an individual who hasn't reached the “age of consent.” Statutory rape is considered a strict liability crime because states don't require the accused to intend, or know, that they were engaging in sexual relations with a person under the age of consent.
2. Most traffic violations are also classified as strict liability crimes. For example, a driver can get a speeding ticket whether or not they intended to, or were even aware that they were speeding. Another example of a traffic offense that doesn't require intent is an overdue parking meter.

In order to be convicted of a crime, the prosecution must prove all the elements in the offence beyond a reasonable doubt..

1. An act or omission forbidden or commanded by law.
2. Violation prevented by sanction of punishment.
3. Wrong doer punished only after following a procedure established by law.

An act to amount to a crime must conform to the following two cardinal principles of criminal liability:

1. No one is held criminally liable unless he had done an act which is expressly forbidden under the criminal law.
2. There is no liability under criminal law for omissions, unless there is a duty imposed by law to do the act