Criminal Law

Omission

Omissions are the opposite of acts; they are the failure to act. Generally, an omission cannot be a crime, but in some circumstances, not acting can satisfy the requirement of actus reus.

A conviction can only be based on an omission when the defendant had a legal duty to act.

Legal Duty to Act

A legal duty to act can be established by any of:

Statute
Relationship
Contract
Voluntary assumption of care
Creation of risk

Bystander

Bystanders usually fall under the general rule of omissions and are not liable for failing to stop a crime. Exceptions may exist if a bystander has a legal duty to act, as in an omission, or if the bystander "encouraged" the crime and thereby became an accomplice.