The Comments below apply to the old web site, which has now been replaced by a much more reliable one at www.familysearch.org.
However if you are dealing with family trees which could have been produced using the old version, and the information is not in the new version the warnings below could be relevant.
While the online International Genealogical Index at familysearch is an invaluable tool it is essential to remember that it was set up to record the ordnances of the Church of Latter Day Saints and has many deficiencies which must always be considered:
(1) It is only an index - further information is often available from the source documents, which should always be checked. In many cases the original document will include vital information which is not in index. Most such sources are available on microfilm or microfiche and can be viewed worldwide at the LDS Family History Centres. (The address of the one nearest to you is available on familysearch.)
(2) It is an incomplete index of what is available. Not all parish registers have been input - for instance when a family group comes to an end check the source details as all that may have happened is that you have come to the end of a register. In particular its coverage after 1837 is weak - because the civil registration indexes are available and are reasonably comprehensive. Many Hertfordshire records are on the British Isles Vital Records Index CD which should also be checked. The LDS microfilm library (catalogue on familysearch) may hold microfilms of registers which have not been indexed, and HALS holds registers (mainly recent) which have not been microfilmed.
(3) It is an incomplete index for baptisms/christenings because if was not legally necessary to have children christened in the Church of England. Some children were not baptised - while many non-conformist records have not survived. This is a serious problem if you are trying to trace non-conformist ancestors in Hertfordshire before 1837. Before the Hardwicke Marriage Act of 1754, which regulated the holding of the marriage ceremony, some marriages will not have taken place in a parish church and so will not have been recorded. In addition it can only index records that survive and this will vary from parish to parish. In some cases early registers have been lost, and in other cases there my be gaps where the register was not properly maintained.
(4) It is an inaccurate record. Spellings of surnames only became reasonably standard during the last 150 years (in part a result of civil registration). When someone was illiterate their name may have been written down it different ways by different people at different times. Some registers are in difficult handwriting, while others are faded and almost illegible. Some indexers were better at reading old handwriting than others while transcription errors can occur in preparing any index. The familysearch software takes some account of such variability - but you must always be on your guard.
(5) [This paragraph applies to the old site, but the new (2011) site does not show you these dubious entries]. It contains erroneous information. If the data comes a transcribed register it is likely to be reasonably accurate. Unfortunately some of information comes from family trees which have not been checked for accuracy. Sometimes you may find many similar references, often rather vague and usually in the 17th century, and these definitely come from family trees. My experience suggests that most are from trees submitted by Americans who often made wild guesses as to where in England their ancestors came from - and that these errors were then copied by others who didn't bother to check sources. - Don't be a fool and never treat unchecked information from anyone as a confirmed fact.
(6) Remember that in general burials are not indexed, and that a significant number of children died in infancy. When you check the baptismal registers it is always a good idea to check the burial registers as well. You don't want to end up as one of the many people who - because they didn't check - end up with a family tree which claims to have a direct ancestor who in fact died in infancy. The National Burial Index contains a number of Hertfordshire parishes for the period 1800-1850.
(7) Always work with a good map. Unlike countries such as Canada and Australia, which had land available, all land in England has belonged to someone for a thousand years or more. [Basically many people emigrated because it was easier than to move within England.] This means that the poorer people needed to have a good reason to move and most marriages involved people from the same village or somewhere within walking distance. Of course there were exceptions for various reasons - but if there is significant (in English terms) distance between say, the place of marriage and the place where children were baptised, be on your guard. See Population Movements in Rural Hertfordshire which shows how little mobility there was at as late as 1881.
(8) Read Right Name, Wrong Body? and beware of assuming that an index entry relates to your ancestor. If your reply comes up with 20 people who match your search criteria you know you have problems. In a county the size of Hertfordshire there may well be two or more couples with the same names - and I have seen erroneous family trees where several different families have been combined as one.
Remember: The IGI is an index of souls saved by Church of Latter Day Saints which also happens to contain information of interest to the genealogist. Some of the information from "Patron Submission Sheets" would appear to be no more than wild guesses based on inadequate information.
See also Sources and Reliability
Finally: Everyone's ancestral lines run out at some stage - usually due to a lack of accessible surviving records. Many people have problems with basic family information in the early 19th century (particularly non-conformist families whose oldest records are the 1841 census, and civil registration after 1837.) Others come adrift in the mid-17th century due to the problems of Civil War and the Commonwealth, including the loss of many earlier parish registers. In many cases the earliest registers or bishop's transcripts only go back to the start of the 17th century - while the earliest registers start in 1538.
See also Where is my ancestor's baptism before 1837?
Page updated July 2006