Immigration is basically a quota system.  For each category of immigrant visas, there is a quota allotting the number of visas that can be used in any given fiscal year.  There are two main categories and several sub categories beneath them.  They are family and employment.  For the most part, the closer the family relationship or the higher educated the employee, the higher their ranking will be in the quota system.  For example, the unmarried adult child of a US citizen currently has about a four and a half year backlog, while the sibling of a US citizen has an eleven year backlog.  To give an example in the employment based aspect, an outstanding professor or researcher has no backlog while a professional holding a bachelor’s degree has four and a half year backlog.  This system can be tracked on line by viewing the State Department’s Visa Bulletin.  It is important to plan ahead properly in order to deal with the system of priority dates.  An attorney will figure a way, if possible, to keep his or her client present in the US in valid non-immigrant status until such time as their priority date becomes current, thereby allowing them to apply for adjustment of status to lawful permanent resident.  For certain countries which send many immigrants to the US, there are even greater backlogs separate from the rest of the world.  Those countries are China, India, the Philippines and Mexico.  There are some borderline cases where an argument can be made (but planned from the beginning) to include a client in a higher preference category.  Earlier this year, this did not seem to matter in the employment based preference system because there were few, if any, backlogs.  Now, the backlogs are significant and it is crucial to take these factors into account before embarking on a new case.