Marriage is considered as a sacrament in Hindus. It is the last of the 10 sacraments in the Hindu religion for regeneration of men and obligatory for every Hindu who does not desire to adopt the life of a sanyasi. The first and foremost condition for a Hindu Marriage is that the both parties should be Hindus. This was held in Gullipilli Sowria Raj vs Bandaru Pavani [AIR 2009 SC 1085]. Hindu Marriage Act, 1955 has laid down few necessary conditions for a valid Hindu Marriage. This is given under section 5 of the Act.

In Lila Gupta vs Laxmi Narain & Ors [AIR 1978 SC 1351], the apex court held that all conditions under section 5 of the Act are not mandatory.

The following are the necessary conditions for a valid Hindu Marriage:

1. Monogamy:
   Section 5(i) of the Act prohibits polygamy and polyandry. It says that neither party should have a living spouse at the time of the marriage. Failure of this condition would make the marriage null and void under section 11 of the Act. Apart from this, the party would be liable for bigamy under sections 494 and 495 of the Indian Penal Code, 1860 and section 17 of the Hindu Marriage Act, 1955. Schedule Tribes are exempted from this but they must have an early and lasting custom for this.
   
   In cases like Bhogadi Kannababu & Ors vs Vuggina Pydamma & Ors [AIR 2006 SC149] and Yamunabai Anantrao Adhav A vs Ranantrao Shivram Adhav & Anr [AIR 1988 SC 644], the apex court held that during the subsistence of first marriage, the second marriage would be null and void.
   
   Offence of Bigamy would be constituted only when the first marriage is solemnized according to proper ceremonies and rituals. This was held by court in cases like Dr. A.N. Mukerji vs State [AIR 1969 All 489] and Santi Deb Berma vs Smt. Kanchan Prava Devi [AIR 1991 SC 816].  2. Mental Capacity: This clause was inserted in the Act through The Marriage Laws (Amendment) Act, 1976. As per the clause, three conditions must be fulfilled as per section 5(ii) of the Act for a valid Hindu Marriage.
    
2. The conditions are:
   neither party, at the time of marriage:
   
   1. is incapable of giving a valid consent due to his/her unsound mind
   2. has been suffering from mental disorder of such a kind or to such an extent as to be unfit for marriage and the procreation of children
   3. has been subjected to recurrent attacks of insanity.
   
   If any of these conditions are not fulfilled then marriage is voidable under section 12(1)(b) of the Act. The clause of epilepsy was removed through The Marriage Laws (Amendment) Act, 2001. In a case in this regard, Allahabad High Court held that the validity of marriage will be nullified only when it is impossible for one party to remain in the marriage due to the unsoundness of the other party.
   
   Before the enactment of the Amendment, in the case of Anima Roy vs Probodh Mohan Roy [AIR 1969 Cal 304], the Calcutta High Court held that since the expression �insane� has not been defined under the Act, its meaning and purport would be the same as under Section 3(5) of the Insanity Act which provides that any person suffering from mental derangement of any kind may be regarded as idiot or insane.
   
   In Smt.Alka Sharma vs Abhinesh Chandra Sharma [AIR 1991 MP 205], the High Court of Madhya Pradesh held that the court can nullify the marriage if either condition or both conditions are fulfilled due to mental disorder of the party.
    
3. Age to the parties:
   At the time of enactment of the Act, the legal age for the marriage of boy and girl was 18 years and 15 years respectively. However, later on, The Marriage Laws (Amendment) Act, 1976, changed the minimum age to 21 years and 18 years respectively. According to the ruling in Pinninti Venkataramana and Anr vs State [AIR 1977 AP 43], breaching of the condition didn�t amount to nullity of the marriage but it is an offence under section 18(a) of the Act i.e. simple imprisonment up to 15 days or a fine of Rs. 1000/- or both. According to section 10 of The Prohibition of Child Marriage Act, 2006, any person performing, conducting, directing or abetting a child marriage shall be punished with rigorous imprisonment up to two years and fine of one lakh rupees.
    
4. Prohibited Degrees of Relationship:
   Section 5(iv) of the Act prohibits solemnization of marriage of persons falling within prohibited degree of relationship. If any marriage is solemnized under this then the marriage would be void under section 11 of the act. Apart from this, violation of this clause would amount to simple imprisonment upto 1 month or a fine of Rs. 1000/- or both under section 18(b) of the act. Section 3(g) defined persons falling within prohibited degree of relationship.
   
   They are:
   
   1. If one is a lineal ascendant of the other; or
   2. If one was the wife or the husband of a lineal ascendant or descendant of the other; or
   3. If one was the wife of the brother or of the father�s or mother�s brother or of the grandfather�s or grandmother�s brother of the other,
   4. If the two are brother and sister, uncle and niece, aunt and nephew, or children of brother and sister or of two brothers or of two sisters.
      
      It also includes:
      
      1. Relationship by half or uterine blood as well as by full blood;
      2. Illegitimate blood relationship as well as legitimate;
      3. Relationship by adoption as well as by blood.
   
   
   In simple words we can say that a man cannot marry either of the following persons:
   
   1. His lineal ascendant
   2. Wife of his lineal ascendant
   3. Wife of his lineal descendant
   4. Brother�s wife
   5. Wife of his father�s brother
   6. Wife of his mother�s brother
   7. Wife of his grandfather�s brother
   8. Wife of his grandmother�s brother
   9. Sister
   10. Sister�s daughter
   11. Father�s sister
   12. Mother�s sister
   13. Father�s sister�s daughter
   14. Father�s brother�s daughter and
   15. Mother�s brother�s daughter.
   
   
   Similarly, a woman cannot marry either of the following persons:
   
   1. Her lineal ascendant
   2. Husband of her lineal ascendant
   3. Husband of her lineal descendant
   4. Brother
   5. Father�s brother
   6. Mother�s brother
   7. Nephew
   8. Sister�s son
   9. Uncle�s son
   10. Father�s sister�s son
   11. Mother�s sister�s son and
   12. Mother�s brother�s son.
   In Shakuntala Devi vs Amar Nath [AIR 1982 P H 221], Punjab and Haryana High Court held that two persons can marry within the prohibited relationship but there should be a proof of established custom i.e. very old and beyond human memory.
    
5. Prohibition of Sapinda Relationship:
   Section 5(v) of the Act, marriage between the persons having sapinda relationship is prohibited unless there is a custom which allows them to do so. Any marriage solemnized under this would be void under section 11 of the Act and violation of this clause would amount to simple imprisonment up to 1 month or a fine of Rs. 1000/- or both under section 18(b) of the act.
   
   Section 3(f)(ii) of the Act says that Two persons are said to be �sapindas� of each other if one is a linear ascendant (i.e. is a blood relative in the direct line of descent � the children, grandchildren, great-grandchildren, etc. of a person) of the other within the limits of �sapinda� relationship, or if they have a common lineal ascendant who is within the limits of �sapinda� relationship with reference to each of them.
   
   According to section 3(f)(i) sapinda relationship extends as far as the third generation (inclusive) in the line of ascent through the mother, and the fifth generation (inclusive) in the line of ascent through the father, the line being traced upwards in each case from the person concerned, who is to be counted as the first generation. If there is any common ancestor of 2 persons then both are sapinda to common ancestor and they would be sapinda of each other.