London Metropolitan Archives
These Archives cover both London and the former County of Middlesex, including some areas which were once in Hertfordshire. In addition many people from Hertfordshire moved to London for work, while many of the larger country hoses in Hertfordshire were owned by people who worked in London. Most of the major transport route through Hertfordshire (roads, railways and canal) start in London, so there was considerable through the county, in addition to agricultural supplies from Hertfordshire farms. In addition a number of large residential schools for children, and mental hospitals were built in Hertfordshire to provide for people from London/Middlesex workhouses. For this and other reasons these archives can prove very useful for some aspects of Hertfordshire Research.
Wikipedia entry giving history, etc., of the Archive.
The document collection of the Guildhall Library is now at the L.M.A.
The Society of Genealogists Library is in easy walking distance of the L.M.A.
See also Middlesex