This research advances an evolution-based model of the function and content of investment in romantic relationships. In developing this model, theory testing has proceeded concurrently with the validation of a multidimensional measure of partner-specific investment (PSI). Ten strategies of investment were identified through factor analysis and scaled to form the PSI Inventory. These scales form a diverse set of investment measures that are reasonably independent, internally consistent, and reliable across raters. Both gender and sociosexual orientation predict use of specific PSI strategies. Overall PSI scores correlate positively with partner’s feelings of love and felt security in the relationship, correlate negatively with degree of sexualizing of others by partner, and do not correlate with the self’s performance of mate retention behaviors. It is concluded that the PSI Inventory possesses adequate psychometric properties and satisfactory convergent, discriminant, and incremental validity, thus suggesting that it constitutes a viable means of assessing individual differences in investment. These investigations show how a multidimensional, adaptationist approach to investment can further our understanding of these individual differences.