The claim of illegitimate children refers to the legal act of transforming illegitimate children into married women through legal procedures. The claim of illegitimate children is generally applicable to cases where illegitimate children cannot be corrected. There are two forms of claim for illegitimate children: voluntary claim and compulsory claim.
 1. Voluntary claim means that the natural father voluntarily recognizes the illegitimate child as his own child. Voluntary claim may arise from the fact that the natural father has expressed his intention or raised him. Voluntary claim is generally required to be carried out in a certain way, which is an important behavior. For example, as stipulated in the Japanese Civil Code, the claim must be reported to the household registration department. For example, as stipulated in the French Civil Code, the claim must be notarized. For some countries, as stipulated in the Swiss Civil Code, the claim must be applied to the identity administrator. For some countries, the claim must be applied to the guardianship court. The act of claiming is a kind of civil act of identity nature, which cannot be represented. However, some countries stipulate that voluntary claim can be made only if the natural father has the intention to claim the illegitimate child as his own child, without performing other forms. If the illegitimate child is a minor, the consent of the mother is required to claim the illegitimate child voluntarily; if the illegitimate child is an adult, the consent of the adult woman is also required to claim the illegitimate child voluntarily.
 2. Compulsory claim means that when the natural father of an illegitimate child does not automatically claim it, the relevant parties will appeal to the court with sufficient evidence, and the court will confirm the blood relationship between the illegitimate child and his/her natural father through legal proceedings. The fundamental purpose of compulsory claim is to punish the party who evades the obligation and protect the legitimate rights and interests of children born out of wedlock. Compulsory claim is based on the premise that there is a blood relationship between the illegitimate child and the claimant. Therefore, the plaintiff should bear the burden of proof to prove the cause and fact of the compulsory claim, such as proving the fact that the illegitimate child's biological father and biological mother live together, or applying for "parent-child identification". The claimant of compulsory claim is generally the natural mother of the illegitimate child or the illegitimate child himself. Some countries also stipulate the statute of limitations for compulsory claims, most of which stipulate one year, while others stipulate two years.