Mens Rea (Culpable Mental State)
"Mens Rea," Latin for the term "guilty mind," refers to the legal requirement that a defendant must have a culpable state of mind before she can be found guilty of a crime. There must be a concurrence between the state of mind and the Actus Reus, i.e. they must occur at the same time. The requisite Mens Rea should apply to each and every element of the crime.
Specific v. General Intent Crimes
At common law, crimes could be classified as requiring general or specific intent. A general intent crime requires a concurrence in time of the mens rea and the actus reus, with the mens rea consisting of an intent to complete the physical acts of a crime. Specific intent crimes require something more than an intent to do the physical acts of a crime. For example, theft is a specific intent crime because in order to be found guilty you must be found to have intended to take an item from another person with the intent to deprive that person of her property permanently. Many courts and some academics have argued that the distinction between general and specific intent crimes is both ephemeral and problematic, and should be abandoned.
Under the Model Penal Code a person acts purposely "(i) if the element involves the nature of his conduct or a result thereof, it is his conscious object to engage in conduct of that nature or to cause such a result; and (ii) if the element involves the attendant circumstances, he is aware of the existence of such circumstances or he believes or hopes that they exist."
Under the Model Penal Code a person acts knowingly "(i) if the element involves the nature of his conduct or the attendant circumstances, he is aware that his conduct is of that nature or that such circumstances exist; and(ii) if the element involves a result of his conduct, he is aware that it is practically certain that his conduct will cause such a result." 
Under the Model Penal Code a person acts recklessly if he "consciously disregards a substantial and unjustifiable risk that the material element exists or will result from his conduct. The risk must be of such a nature and degree that, considering the nature and purpose of the actor's conduct and the circumstances known to him, its disregard involves a gross deviation from the standard of conduct that a law-abiding person would observe in the actor's situation." 
Under the Model Penal Code a person acts negligently "when he should be aware of a substantial and unjustifiable risk that the material element exists or will result from his conduct. The risk must be of such a nature and degree that the actor's failure to perceive it, considering the nature and purpose of his conduct and the circumstances known to him, involves a gross deviation from the standard of care that a reasonable person would observe in the actor's situation." (emphasis added). Sometimes also referred to as gross negligence, criminal negligence should be distinguished from civil negligence in that criminal negligence requires a gross deviation from the normal standard of care.
Strict Liability Crimes
In certain cases, the legislature has decided that no mental state is required to prove a crime. The prosecution is not required to prove any mental state at all. These are called strict liability crimes. Often these are minor offenses involving public safety or regulated industries. The most common strict liability crimes in the United States include parking tickets, sale of alcohol to a minor and statutory rape. Some academics have distinguished strict liability crimes by arguing that these are often Malum Prohibitum (wrong because they are prohibited) but not Malum In Se (wrong in itself). Statutory rape, however, is the one crime that does not fit this distinction very well. In that case, the legislature has made a policy decision to eliminate a burden of proof on the prosecution, making the crime easier to prosecute. The Model Penal Code restricts strict liability crimes to violations and statutory rape of a girl under the age of ten.
A defendant who acts with an intent to kill individual A but accidentally kills individual B may still be found guilty of murder with intent to kill under the theory of transferred intent. This doctrine is also explained as "intent follows the bullet". The defendant may not even have to be aware the actual victim existed. Thus, the doctrine applies when a defendant fires a weapon at an intended victim and that bullet penetrates a nearby apartment, killing an actual victim. Transferred intent will generally apply to any case in which the required state of mind is either intent or knowledge. However, if the crime is committed with recklessness or negligence, the doctrine is not as clearly established. In these cases, the defense should argue that the doctrine either does not apply, or that a close connection between the intended and actual victim must be proven by the prosecution.