Their Function
The Sages extrapolated from the words of the Scriptures1 that all matters pertaining to the transacting of marriage and divorce must be effected in the presence of two witnesses. These witnesses will be able to testify regarding the marital status of the couple, if doubt in this area ever arises; but more importantly, they actually effect the marriage (or divorce). According to Jewish law, the two witnesses play the most pivotal role in the marriage ceremony.
The involvement of witnesses is needed several times during the course of the wedding ceremony. The same two witnesses can perform all the different functions,2 or different sets of witnesses can be used — which is traditionally the case, due to the desire to bestow honors on as many individuals as possible.
The following parts of the ceremony require witnesses:

• According to Jewish law, the witnesses play the most pivotal role in the marriage ceremonyThe tena’im (engagement contract) must be signed by two witnesses.3
• The ketubah (marriage contract) must also be signed by witnesses.
• The witnesses who actually effect the marriage are the ones who stand beneath the chupah and witness the kiddushin (betrothal) — which occurs when the groom places the ring on the bride’s finger — and hear the betrothal words which he utters at that moment.
  After the chupah, these two witnesses follow the bride and groom to the Yichud Room, where they ascertain that there is no one in the room besides the bride and groom, and observe the door being shut and locked. They then wait outside the room for the amount of time halachically necessary for the couple to remain secluded.

Qualifications
For all the aforementioned functions, male, Torah-observant adults over the age of bar mitzvah are required.
Relatives of the bride or groom cannot serve as witnesses. The following relatives of either the bride or groom are disqualified from being witnesses: a father or step-father; grandfather or step-grandfather; great-grandfather, etc.; sons and sons-in-law; grandsons and grand-sons-in-law; brothers and brothers-in-law; uncles and great-uncles (by blood or marriage); cousins (by blood or marriage); and nephews or great-nephews (by blood or marriage).
By the same token, the two witnesses cannot be related to each other.
Because the wedding ceremony is attended by family members and others who are not qualified to be witnesses, the officiating rabbi or master of ceremonies customarily announces the names of the two selected witnesses; while emphasizing that only these two are serving as witnesses, to the exclusion of all others in attendance.

Their Function
The Sages extrapolated from the words of the Scriptures1 that all matters pertaining to the transacting of marriage and divorce must be effected in the presence of two witnesses. These witnesses will be able to testify regarding the marital status of the couple, if doubt in this area ever arises; but more importantly, they actually effect the marriage (or divorce). According to Jewish law, the two witnesses play the most pivotal role in the marriage ceremony.
The involvement of witnesses is needed several times during the course of the wedding ceremony. The same two witnesses can perform all the different functions,2 or different sets of witnesses can be used — which is traditionally the case, due to the desire to bestow honors on as many individuals as possible.
The following parts of the ceremony require witnesses:

• According to Jewish law, the witnesses play the most pivotal role in the marriage ceremonyThe tena’im (engagement contract) must be signed by two witnesses.3
• The ketubah (marriage contract) must also be signed by witnesses.
• The witnesses who actually effect the marriage are the ones who stand beneath the chupah and witness the kiddushin (betrothal) — which occurs when the groom places the ring on the bride’s finger — and hear the betrothal words which he utters at that moment.
  After the chupah, these two witnesses follow the bride and groom to the Yichud Room, where they ascertain that there is no one in the room besides the bride and groom, and observe the door being shut and locked. They then wait outside the room for the amount of time halachically necessary for the couple to remain secluded.

Qualifications
For all the aforementioned functions, male, Torah-observant adults over the age of bar mitzvah are required.
Relatives of the bride or groom cannot serve as witnesses. The following relatives of either the bride or groom are disqualified from being witnesses: a father or step-father; grandfather or step-grandfather; great-grandfather, etc.; sons and sons-in-law; grandsons and grand-sons-in-law; brothers and brothers-in-law; uncles and great-uncles (by blood or marriage); cousins (by blood or marriage); and nephews or great-nephews (by blood or marriage).
By the same token, the two witnesses cannot be related to each other.
Because the wedding ceremony is attended by family members and others who are not qualified to be witnesses, the officiating rabbi or master of ceremonies customarily announces the names of the two selected witnesses; while emphasizing that only these two are serving as witnesses, to the exclusion of all others in attendance.