The Petition
Virtually every application for permanent residence requires an associated immigrant visa petition. The petitioner is different depending on the basis for the request. In the case of family petitions, the family member holding status in the U.S. (the "petitioner") requests the petition on behalf of the beneficiary. In most employment based applications the employer petitions on behalf of the beneficiary. There are other application types that permit the beneficiary to petition on their own behalf. Who "owns" the petition can be important since they hold the right to appeal a denial, not the foreign national.
Immigrant Visa Backlogs
Although there are some types of applicants for whom an unlimited number of visas are issued in a year; many categories of applicant must wait for a visa to become available. Large numbers of applicants for certain immigrant visa kinds have resulted in a backlog of applicants waiting their turn to receive residency. It is wise to learn whether your category is backlogged or not before applying.
In cases where backlogs exist, the date on which the visa petition was received by the immigration service becomes the "priority date" for the application. The Department of State's Visa Bulletin indicates which date the immigration service is currently issuing visas for and can give some indication of projected wait times.
Green Card Rescission/Revocation
Once granted residency there are still events that can result in the loss of the green card. Two common causes of the loss of residency are abandonment and criminal convictions. A resident who spends more than 180 days out of any given year risks being accused of having abandoned their residency. This can be prevented by requesting a reentry permit prior to departure.
Criminal convictions may also result in the revocation of a green card. Even some civil offenses or admissions that don't lead to a conviction can endanger an alien's resident status. There should be particular concern about felonies and crimes involving a moral element such as fraud, theft, sex offenses, etc. Criminal defense attorneys may not be aware of the immigration consequences of certain pleas and admissions. Retaining an immigration attorney to assess the risk to your residency when you are accused of a crime is highly advisable.