The annual investment allowance (AIA) was introduced as part of the Budget 2007 business tax reforms. The AIA will benefit all those small businesses that make capital investments, regardless of their legal form. There are around 4.3 million small businesses, 2.1 million of which are estimated to be making capital investments. For 95 per cent. of these, the AIA will mean that all of their capital expenditure can be written off in the year of investment.