Almost all of the census records are available. A few have been destroyed by fire or other causes. As an example, there is very little left of the 1890, but it is important to check if your family lived in the area that remains. Don't let anyone tell you it doesn't exist. In those cases and where possible, these missing censuses have been replaced by other appropriate records such as tax lists for the area.
Indexes exist for almost all of the federal censuses.
Some were done by the government, others by private organizations or businesses. Both of these indexes can look confusing unless you know how to use them. 1920 - Soundex Soundex is based on the sound of words. The name Smith and Smyth would be coded alike. In this way, you will hopefully not miss names with unusual spellings.