The "Big Four" are the four largest accounting firms consisting of Deloitte, EY, KPMG and PwC. They conduct bookkeeping, auditing and tax planning for their client companies such that they comply with both accounting and legal standards. Currently the Big Four account for the audits of 99% of the companies in the FTSE 100 Index, and 96% of the companies in the FTSE 250 Index.