Criminal defense refers to the rebuttal and defense carried out by criminal suspects, defendants and their defenders to prove that the criminal suspects and defendants are innocent or guilty, and to protect their legitimate rights and interests. Its essence is to give the defendant an opportunity to speak for himself, so that he can participate in the criminal proceedings "meaningfully" and "effectively" as a subject. Through criminal defense, the exercise of defense rights plays a beneficial role in the formation of the judge's final judgment. The right of defense is the litigation right enjoyed by criminal suspects, defendants and their defenders in accordance with the law to defend the investigation and accusations against criminal suspects and defendants. It targets offensive accusations and is the defendant's most basic and core litigation rights. It is the basis for the formation of the criminal defense system. It is impossible to have a criminal defense system without recognizing the right of defense of criminal suspects and defendants. The criminal defense system is a general term for a series of rules regarding defense rights, defense types, defense methods, defender scope, defender responsibilities, defender rights and obligations, etc. The defense system is the guarantee of the right to defense. It is a variety of defense systems established to ensure that criminal suspects and defendants fully and correctly exercise their right to defense.