Caroline Ceniza-Levine Contributor

There are many reasons you could be underpaid despite being excellent at what you do. You might have taken on additional responsibilities but without a commensurate raise. You might be a longtime employee getting small cost-of-living adjustments while new hires coming from the outside leapfrog into higher salary bands. You might be starting out in your career or ramping back up after an absence and only have lower-paying or unpaid projects in your recent history. In all of these cases, in order to get your salary back up to market value, you will need to negotiate a higher starting salary or raise than you have received before.

http://www.forbes.com/sites/carolinecenizalevine/2015/03/06/underpaid-four-salary-negotiation-strategies-if-you-are-currently-paid-below-market/