A recent report from Verizon, presented at this year's RSA conference, contained some interesting information: More than half of the data breaches discovered by organizations weren't found out until months or even years after the breaches actually took place. The data was based on cases investigated by Verizon's incident response team.
More startling was the conclusion that the vast majority of breaches - more than 90 percent - were the result of external attacks. This is contrary to reports from other security sources.
You can read more about the Verizon report here: