In common-law jurisdictions, administration of an estate on death arises if the deceased is legally intestate, meaning they did not leave a will, or some assets are not disposed of by their will.

Where a person dies leaving a will appointing an executor, and that executor validly disposes of the property of the deceased within England and Wales, then the estate will go to probate. However, if no will is left, or the will is invalid or incomplete in some way, then administrators must be appointed. They perform a similar role to the executor of a will but, where there are no instructions in a will, the administrators must distribute the estate of the deceased according to the rules laid down by statute and the common trust.

Certain property falls outside the estate for administration purposes, the most common example probably being houses jointly owned that pass by survivorship on the first death of a couple into the sole name of the survivor. Other examples include discretionary death benefits from pension funds, accounts with certain financial institutions subject to a nomination and the proceeds of life insurance policies which have been written into trust. Trust property will also frequently fall outside the estate but will depend on the terms of the trust. 

Intestacy is the condition of the estate of a person who dies without having in force a valid will or other binding declaration. Alternatively this may also apply where a will or declaration has been made, but only applies to part of the estate; the remaining estate forms the "intestate estate". Intestacy law, also referred to as the law of descent and distribution, refers to the body of law (statutory and case law) that determines who is entitled to the property from the estate under the rules of inheritance.

A testator (/tɛsˈteɪtɔːr/) is a person who has written and executed a last will and testament that is in effect at the time of his/her death. It is any "person who makes a will."