From "The History of the Equals Sign", at The Science Show:
In 1543, [Robert] Record published The Ground of Arts, the first ever maths book in English, which ran through over fifty editions … Until 1557, mathematicians had finished off a calculation by laboriously writing out the words, is equal to, which was sometimes abbreviated to AE or OE from the Latin word for equal, aequalis. But Record had a better idea, why not use a symbol, he said, to avoid, as he put it, the tedious repetition of these words he proposed the use of a pair of parallel lines. Using this simple device that we now call the equals sign released an enormous logjam in the efficient handling of numbers and the implications extended far beyond pure maths.