This book deals with substantive criminal law. Substantive criminal law refers to the physical and mental element (if any) that has to be proved for each criminal offence. It also includes the general principles of intention and causation, the defences available and other general rules such as those on when participation in a crime makes the person criminally liable. Substantive criminal law does not include rules of procedure or evidence or sentencing theory and practice. However, these are equally important parts of the criminal justice system. This chapter, therefore, gives some background information on criminal law. The purpose of the criminal law is considered, as well as how we know what is recognised as a crime, and the sources of criminal law. There are also brief sections explaining the courts in which criminal offences are tried, and the purposes of sentencing. The penultimate section of this chapter explains the burden and standard of proof in criminal cases. The final section looks at the effect of human rights law on criminal law.