Improved reproductive performance has a substantial benefit for dairy cattle farming by decreasing insemination costs and veterinary treatment, shortening calving intervals, and extending the longevity of dairy cows. Unfortunately, the low heritability of classical fertility traits derived from calving and insemination data makes genetic improvement by traditional animal breeding slow. Therefore, there is an interest in finding novel measures of fertility that have a higher heritability, or using genomic information to aid genetic selection for fertility. In my thesis, I showed that “novel” endocrine fertility traits derived from in-line milk progesterone concentrations are more heritable than classical fertility traits. I also showed that in-line milk progesterone combined with genomics is the optimum way to improve selection to improve the underlying biology for fertility in dairy cows. Nonetheless, for an economic breeding goal, classical traits are effective since they are recorded on a much larger scale.