A person's next of kin (NOK) may be that person's spouse, adopted family member or closest living blood relative. Some countries, such as the United States, have a legal definition of "next of kin". In other countries, such as the United Kingdom, "next of kin" may have no legal definition and may not necessarily refer to blood relatives at all.

In some legal systems, rights regarding inheritance (which imply a decision-making capacity — for example, in a medical emergency — where no clear will or instructions have been given, and where the person has no spouse) flow to the closest relative (regardless of the age, with a representative appointed if a minor), usually a child, a parent or a sibling. However, there are people without any close adult relatives and, in such a case, decision-making power often flows to a niece or nephew, first cousin, aunt or uncle, or grandparent.

For example, if a person dies intestate, the laws of some jurisdictions require distribution of the estate to the decedent's spouse or children. However, if there are none of these, the estate can often be distributed to the next closest group of living relatives, whether they be parents, grandparents, first cousins, aunts and uncles, or second cousins in extreme cases. If a person dies intestate with no identifiable next of kin, the person's estate generally escheats (i.e., legally reverts) to the government.

In cases of medical emergency, where a person is incapable (either legally because of age or mental infirmity, or because they are unconscious) of making decisions for themselves and they have no spouse or children, the next of kin may participate in medical decisions made by medical personnel, subject to the specific laws of the jurisdiction.

The inability of persons who are not in a legal marriage to make decisions with respect to the care of a live-in partner has resulted in many jurisdictions giving live-in partners rights equivalent to a spouse in such situations even though most jurisdictions still do not require non-spouses to be made beneficiaries of estates (most jurisdictions either prohibit disinheriting a surviving spouse or grant a surviving spouse the right to "elect against" a will and receive money or property of up to a specified fraction of the estate's value). The inability of same-sex partners to have rights with respect to a partner's medical care or funeral arrangements over and above those of the next-of-kin is one of the main reasons behind litigation to require same-sex marriage or its equivalent.

For the purposes of next of kin, adopted children are treated as blood relatives. However, relatives by marriage other than an individual's spouse are never considered next of kin.

"American statutes typically provide that, in absence of issue and subject to the share of a surviving spouse, intestate property passes to the parents or to the surviving parent of the decedent". Under the civil law system of computation and its various modified forms that are widely adopted by statute in the United States, "a claimant's degree of kinship is the total of (1) the number of the steps, counting one from each generation, from the decedent up to the nearest common ancestor of the decedent and the claimant, and (2) the number of steps from the common ancestor down to the claimant." "The claimant having the lowest degree count (i.e., the nearest or next of kin) is entitled to the property." "If there are two or more claimants who stand in equal degree of kinship to the decedent, they share per capita."

Thus, the following conditions determine the usual order of precedence: