There has been a census every ten years since 1801 (excluding 1941) but only those returns after 1841 (with a few earlier exceptions) carry details of named residents. census returns for 1841-1911 can be consulted at the National Records of Scotland in Edinburgh (see also the GRO tutorial). Copies on microfilm may be consulted in LDS Family History Centres around the world. LDS centres also carry microfiche indexes to the 1881 census returns.
SCROL - Scottish Census Results On Line gives access to statistical information from the 2001 census.
Computerised indexes for 1841, 1851, 1861, 1871, 1881, 1891 and 1901 censuses are available at the National Records of Scotland in Edinburgh, and they are also available online (for a fee) at ScotlandsPeople (includes images of the 1861, 1871, 1891 and 1901 censuses, and full transcript of 1881). Index gives surname, forename, age, registration district name and number, enumeration district and page number (i.e. leads directly to full household entry on microfilm copy of census), and facsimiles (or transcript, for 1881) of census returns are available online.
Ancestry.co.uk now have transcriptions/indexes (but no images) of the 1841, 1851, 1861, 1871 and 1891 Scottish census returns.
FreeCen Scotland is a project indexing 19th Century census records and putting them online with free access, starting mainly with 1841 and some 1851 records. It is part of UK Census Online. Indexes for the UK or any individual place or county can be searched from the FreeCen Search page.
Scotland census finder is a tool designed to help you find sites which offer free census records online for Scotland. It has extensive lists of census indexes, organised by county. It is part of the censusfinder site of Brenda Hay.
A very useful listing of Microfilm Numbers of Scottish Census Records provided by David Wills.
The 3rd edition of Census Records for Scottish families at Home and Abroad by Gordon Johnson has been published by Aberdeen & N.E.Scotland FHS, ISBN 0 947659 74 9, vi,100pp. The book, which can be purchased from the society directly, includes an outline of census records in many other countries and a directory of census holdings in Scottish libraries and archives, updated to include changes resulting from the 1996 local government reorganisation.
Peter Ruthven-Murray's information leaflet entitled Scottish Census Indexes, 1841-1871 is published by and available from the Scottish Association of Family History Societies.