Fault liability theory in criminal law refers to the legal principle that a person can only be held criminally responsible for an offence if they acted with a certain degree of fault or culpability, such as intention, recklessness, or negligence. This theory is based on the concept that punishment should only be imposed if an individual is morally blameworthy for their actions. In the UK, this typically means that the prosecution must prove both the physical (actus reus) and mental (mens rea) elements of the crime. As such, fault liability ensures that a person's state of mind and knowledge of their actions are taken into account when assessing their criminal responsibility.