A petition for a declaration of nullity is an accusation that one's marriage was invalid due to some factor present on the day when marital consent was exchanged (i.e. at the time of the wedding). Consequently, a petition for a declaration of nullity must include the reason for petitioning, that is, the ground(s) of nullity on which the marriage is being challenged. There are three major causes of nullity, each of which can be further classified into several distinct grounds. A marriage can be invalid when there is 1) a defect of consent, 2) the presence of an impediment which rendered one or both of the parties unqualified for marriage, or 3) a defect in the form of celebration. Below is a list of the grounds of nullity and a brief description of each ground.

 DEFECTS OF CONSENT

Consent is what makes marriage. However, certain people are not capable of truly giving consent (either permanently or temporarily) because of some psychological obstacle, error, or lack of freedom. Others are capable of giving consent, but they purposefully withhold something essential from their consent or attach their consent to some external factor.

• Lack of Sufficient Use of Reason (c. 1095, 1º) Permanent or temporary lack of the use of reason due to a severe disability, mental illness, psychological disturbance, or intoxication at the time of the celebration of marriage prevents a person from making an act of consent.
• Grave Defect of Discretion of Judgment Regarding the Essential Matrimonial Duties (c. 1095, 2º) In order to be able to consent to marriage, a person must possess at least a minimal ability to make a concrete judgment about the essential rights and duties of the marital covenant (i.e. permanence, fidelity, and openness to children). Substance abuse, psychological disorders, severe emotional imbalance, trauma, extreme immaturity, and other similar factors present at the time of consent can deprive a person of the ability to reason about the decision to marry. Factors like youth, pregnancy, or moderate pressure do not typically result in a grave lack of discretion of judgment on their own.
• Incapacity to Assume the Essential Obligations of Marriage (c. 1095, 3º) At the time of the celebration of marriage, one of the parties is altogether incapable of taking on the essential obligations of marriage (i.e. permanence, fidelity, and openness to children) for reasons of a psychological nature. The psychological problem must be so severe that it makes it humanly impossible, not merely difficult, to take on the essential obligations of marriage. This impossibility can be caused by a mental illness, a psychological disorder, a severe addiction, or a strong homosexual inclination.
• Ignorance of the Conjugal Act (c. 1096) For matrimonial consent to be valid, it is necessary that the parties at least be not ignorant that marriage is a permanent partnership between a man and a woman which is ordered toward the procreation of offspring by means of some sexual cooperation. This ground addresses the basic knowledge needed to marry, and thus, the Church presumes that those who have reached the age of puberty have the necessary knowledge to marry.
• Error of Person (c. 1097 §1) At the time of marriage, one of the parties believes he/she is marrying one person, when in fact, it is someone else.
• Error Concerning a Quality of Person (c. 1097 §2) Error about a certain quality of a person, such as a personality trait or some fact in the person's history, does not make a marriage invalid unless that quality is directly and principally intended. In other words, the quality is desired more than the actual person.
• Fraud or Imposed Error (c. 1098) A person's consent is invalid if it is given as a result of being deceived by fraud that (1) is intentionally inflicted in order to get that person to give consent and (2) concerns some quality of the other party which of its very nature can seriously disturb the partnership of marriage. The deceit can be perpetrated by the other party or even by some third party, such as a parent.
• Error Concerning Marriage (c. 1099) Normally, error about unity (i.e. marriage to one person), indissolubility (i.e. marriage is for life), or sacramental dignity (i.e. marriage between the baptized is a sacrament) of marriage does not make a marriage invalid. Error of this kind only invalidates when it determines the will. In other words, the error is so deeply held that, in the person's mind, the only kind of marriage that exists is unfaithful marriage, or dissoluble marriage, or non-sacramental marriage. The alternative never seriously enters his/her mind.
• Total Simulation (c. 1101 §2) One or both parties has no intention to establish any kind of marital partnership, but has some totally unrelated goal that can be achieved through the appearance of marriage. The ceremony itself is a sham.
• Partial Simulation Against the Good of Children In spite of what the parties say in their marriage vows, one or both of the parties actually wills against one of the essential elements of marriage, in this case, the good of children. By its nature, marriage is directed toward bearing and raising children. By marrying, spouses give each other the right to procreative acts, even if those acts cannot or do not end up resulting in pregnancy due to infertility, age, etc. If, at the time of marriage, a party excludes this right, the party marries invalidly.
• Partial Simulation Against the Good of Fidelity In spite of what the parties say in their marriage vows, one or both of the parties actually wills against one of the essential elements of marriage, in this case, the good of fidelity. As terrible as adultery is, it does not make a marriage invalid. But, if a party enters marriage while intending to be unfaithful, that intention makes the marriage invalid. In other words, the party must consider bigamy or adultery to be a right.
• Partial Simulation Against the Good of Permanence In spite of what the parties say in their marriage vows, one or both of the parties actually wills against one of the essential elements of marriage, in this case, the good of permanence. Marriage lasts until death. No matter how bad things get, even if the parties have to separate, the bond of marriage remains, making a second marriage impossible. However, if a party enters marriage while reserving the right to dissolve the marriage through divorce and potentially remarry, that party marries invalidly.
• Partial Simulation Against the Good of the Spouse In spite of what the parties say in their marriage vows, one or both of the parties actually wills against one of the essential elements of marriage, in this case, the good of the spouse. By its nature, marriage is directed to the good of the spouses. When spouses marry, they agree to enter a partnership that is ordered toward their mutual good. If one of the parties actually intends to enter a marriage that is ordered toward the harm or corruption of the other party, the party marries invalidly.
• Conditioned Consent (c. 1102) One or both parties consent to marriage if and only if a certain condition is fulfilled. If the condition is not fulfilled at the time of the marriage, or if the condition relates to the future, the marriage is invalid.
• Force or Fear (c. 1103) When a person experiences force or grave fear inflicted even unintentionally by some other person(s), and this force is so great that the person feels compelled to choose an otherwise undesired marriage to be rid of the force or fear, then the marriage is invalid.

IMPEDIMENTS

Impediments disqualify one or both parties from entering marriage. In general, the Church is extremely diligent about discovering impediments before marriage, but sometimes they can remain undiscovered until after the marriage. Note that certain impediments only apply to marriages in which at least one party is Catholic.

• Age (c. 1083) A woman under 14 years of age and a man less than 16 years of age cannot validly marry. If neither party is Catholic, then civil law determines the minimum age for a valid marriage, but never below the age of puberty.
• Antecedent and Perpetual Impotence (c. 1084) A person who is permanently, medically incapable of performing the sexual act cannot validly marry. This is not to be confused with simple infertility or even total sterility, which do not make a marriage invalid.
• Prior Marriage (c. 1085) Marriage is for life. If either of the parties has previously been validly married, and the previous spouse is still living, there can be no second marriage. Note, however, that if the first marriage is later discovered to have been invalid, there was never any impediment to the second marriage.
• Disparity of Worship (c. 1086) Without a dispensation from the Church, a Catholic cannot validly marry a person who has never been baptized. This impediment is often dispensed, but sometimes the need for dispensation is overlooked. Note that this impediment only applies to marriages between Catholics and non-baptized persons, and not to (1) marriages between baptized non-Catholics and non-baptized persons, nor to (2) marriages between Catholics and baptized non-Catholics.
• Sacred Orders/Vow (cc. 1087-1088) A man who has been ordained to the diaconate or the priesthood cannot validly marry. A man or woman who has taken a public, perpetual vow of chastity in a religious institute cannot validly marry.
• Abduction (c. 1089) A man cannot validly marry a woman whom he has kidnapped with the intention of marrying her.
• Crime (c. 1090) A person who brings about the death of his or her own spouse or somebody else's spouse in order to marry is impeded from getting married.
• Consanguinity, Affinity, Public Propriety, and Adoption (cc. 1091-1094) Canon law prohibits marriages between certain close relatives (consanguinity), as well as certain in-laws (affinity). It also prohibits marriage between a person and the parent or child of someone with whom that person has publicly cohabited, even if they were not married or were only invalidly married (public propriety). Finally, it prohibits marriage between certain people related by adoption.

DEFECTS OF FORM

Catholics are required to marry according to the form specified by the Church, namely, the presence of a priest or deacon who asks for and receives a manifestation of consent, and in the presence of at least two additional witnesses. Very often, the Church authority grants a dispensation allowing for some other form of celebration, such as marriage by a Protestant minister. But when there has been no dispensation, Catholics do not marry validly unless they marry according to the required canonical form.

• Lack of Form When a Catholic, even a non-practicing Catholic, attempts to get married outside of the Church without a dispensation (e.g., before a justice of the peace), the marriage is invalid.
• Defective Form When the form of marriage is generally observed, but some required element is missing, the marriage is invalid.