This is a common question people ask us, but it isn't a simple question. Many people would say a dehumidifier, and while they would not be wrong, they definitely would not be completely right.
To control moisture in a crawlspace, you must prevent it from entering in the first place. We do this through CleanSpace, a vapor barrier that is sealed when installed.
The second step is to install a SaniDry Sedona dehumidifier. Now that most of the moisture is stuck outside the crawlspace, the dehumidifier can help to regulate the humidity level.