The early English censuses (1801-1851) provide a mine of information about England in the early nineteenth century but the totals which they contain needs to be treated with care. They contain many arithmetical and printing errors, a significant number of returns are missing, and a unit with the same name in successive censuses might have changed boundaries. There are also problems of coverage. Infants and young children were substantially under-reported and men in the army, navy, and merchant marine were not included, which affected the censuses of the Napoleonic war period significantly. Moreover, in 1851 the basic units of the census were changed. Where previously totals for the parish, the hundred and the county were returned, in 1851 the units were the parish, the registration sub-district, the registration district, and the registration county. Even the national unit changed. Dozens of Welsh parishes became English and an even larger number of English parishes became Welsh.The various sources of error, deficiency, and incomparability are described in the book and two series of tables are provided. The first series uses the ''old'' units of the first five censuses; the second uses the ''new'' units created in 1851. For both series male, female, and total populations are given for all six censuses at each census date. Each series exists in two versions. The first uses the totals to be found in the census volumes, corrected only for arithmetic and printing errors. The second carries the corrections further to offset the under-recording of infants and young children, the absence of men in the army, navy, and merchant marine, and to overcome the problems posed by the absence of men in militia camps on census night. Some tables are very large. The table for the smallest units consists of almost 15,000 places. The large tables are not printed but are provided on an accompanying CD.