This index (which you can also visit in person at the Family Record Center described two paragraphs below) is one of the most significant single resources for genealogical research back to Victorian times.
Note, however, that significant penalties for failure to register births, marriages or deaths were introduced only in the 1870s: prior to that, it does appear that not everyone bothered to comply with the civil registration requirements.
We do not vouch for the accuracy of information on other sites.
The guidance given on this page is based on our experience as amateur genealogists who have used the internet and have found it both helpful but not always reliable (like Burke's Peerage, Landed Gentry, etc, qv below).
Once completed, the Free BMD project will provide on-line birth/marriage/death index information for the period 1837-1983, and increasingly it already does so for most years between 18.