Following is an excerpt from Rediscovering Passenger Lists.
Since 1820, over 60 million persons have arrived at more than 100 different ports in the United States, bringing with them unique cultures, histories, and family stories. Fortunately, the United States (and to a much lesser degree, Canada) has an excellent collection of documents about this pivotal event in history. Passenger lists, the first tangible evidence of many families in America, exist for probably 90 to 95 percent of immigrants.
For about two hundred years, from 1607/20 to 1820, colonists and other immigrants arrived at the shores of North America with few requirements to register their arrival. Most immigrants were citizens of the country to whose colonies they were migrating (e.g., British to English colonies, French to Quebec, Dutch to New Netherlands, Spanish to Florida, etc.). In 1819, Congress passed a law regulating the number of passengers, based upon the total tonnage of the ship. Although this did little to improve the living conditions for passengers, the law mandated the keeping of a list of arrivals, which has been a godsend for family historians. The Bureau of Customs was charged with keeping the passenger lists. Therefore, the lists from 1820 through about 1891 (the ending dates vary by port) are called the "Customs Passenger Lists." The Bureau provided blank forms to the shipping companies, which the captains or their mates prepared onboard. The forms were then submitted to the collector of customs at the port of arrival.
Although passengers arrived at about 100 different ports over the years, most ports saw only infrequent traffic. Sometimes just a couple of ships would arrive in a given year. And during the course of these early years, most of the immigration traffic tended to be directed to one of five major ports: Boston, New York, Philadelphia, Baltimore, and New Orleans. Although Philadelphia had been the most popular of these ports during the colonial era, within the first two decades of federal immigration regulation, New York emerged as the preferred port of arrival. By 1850, more immigrants arrived in New York than in all other ports combined.
New forms were instituted at different times in different ports depending upon various factors, such as who was in charge of the port. Some ports were immediately regulated by federal immigration officials (in 1891 when the Immigration Bureau, now the INS took over), while other ports were regulated by local officers contracted by federal officials. Typically, any lists created under the authority of the Immigration Bureau are considered Immigration Passenger Lists, even though ports may have begun keeping them at various times.
The pre-1891 lists that survived the ravages of time eventually became the custody of the National Archives, where they were microfilmed for preservation and improved access. Thus we have a fairly complete collection of documents that identify more then 18 million persons who arrived in the United States during the jurisdiction of the Bureau of Customs. Virtually all of the post-1890 lists survived and were eventually acquired by the National Archives after its creation in 1935. Studies show that about 6 percent of the lists are difficult or impossible to read, with that number reaching as high as 15 percent for the pre-1902 lists.