The purpose of this paper is to review research about mastitis and milk quality on organic dairy herds. Mastitis is consistently the most commonly reported disease on both ORG and conventional (CON) dairy farms. In the US, concern about udder health of cows on ORG herds is often related to the blanket prohibition of antimicrobial usage. Cows on ORG herds that develop mastitis are usually treated with a variety of alternative compounds such as whey-based products, herbals, and essential oils. Although none of these products have any proven efficacy, ORG dairy farmers are able to successfully manage mastitis. Several studies have compared bulk tank SCC and rate of clinical mastitis between herds managed using ORG or CON systems. Although most studies have shown no difference in bulk tank SCC between management systems, some studies have reported higher bulk tank SCC for ORG as compared with CON herds. ORG herds are typically smaller than CON herds and studies that have accounted for herd-size have not reported significant differences in bulk tank SCC or other measures of milk quality. Management practices used to achieve low bulk tank SCC have slightly varied. In one study a greater proportion of lactating cows in ORG herds had milk withheld from sale as compared with cows in CON herds. In contrast to bulk tank SCC, fewer cases of clinical mastitis are generally reported in cows on ORG as compared with CON farms. A recent study reported 0.21 versus 0.31 cases of clinical mastitis per 305-cow-days for cows in ORG versus CON herds. Within the framework provided by NOP regulations, the size and intensity of management of ORG dairy herds varies considerably. Milking practices used on ORG farms are very similar to practices used on similarly sized CON farms. Several management practices such as use of milking gloves and a quarantine unit to isolate infected cows have been shown to result in improved udder health and milk quality for ORG herds. The use of external consultants has also been associated with improved milk quality. In summary, both ORG and CON herds that use well-known mastitis prevention practices and consult with qualified advisors can minimize mastitis and produce high quality milk.