Criminal law is extending its boundaries to capture conduct that was previously described as civil or regulatory in nature. For example, in some jurisdictions public nuisance, trespass, throwing things at a sporting match, photographing people in private places without their consent and BASE jumping from a building, are criminalised. The unruly nature of criminal law is a serious problem for law makers who need to know what conduct should be criminalised and what conduct should not be criminalised to inform the scope of future criminal laws. It is also a serious problem for members of the community who need to know the minimum standards of behaviour. The unruly nature of criminal law has occurred because several principles underpin the decision to criminalise conduct. The unruly nature of criminal law has not occurred because the decision has been based on the toss of a coin. Rather than recommending the shrinking of the criminal law to tame it, this paper explores the principles underpinning the decision to criminalise conduct. Such principles include harm, immorality, community welfare, individual autonomy and the politics of lawmaking. Analysing these principles will result in a greater understanding of the decision to criminalise conduct. To further understand the unruly nature of criminal law, this paper will contrast criminal wrongdoing from civil wrongdoing from the perspective of the wrongdoing and compensation distinction, public and private distinction, and the essentialist distinction. Making these contrasts will help determine where to draw the boundaries of criminal conduct.