Methods to increase Campbell's (1957) internal and external validity as well as Cook and Campbell's (1979) construct and conclusion validity are reviewed. For internal validity or valid causal inference, designs and methods to draw causal conclusions from nonrandomized studies are considered. Greater collaboration between the causal inference and structural equation modeling traditions would benefit both. For external validity, generalizing results, treating partners and studies as well as participants as random is strongly encouraged. For construct validity, particularly the psychological meaning of measures, multivariate models that treat measures from both overtime and dyadic data as being a combination of multiple constructs are discussed. For conclusion validity or valid statistical inference, the problem of low power when generalizability is high and the assumption of independence are discussed. Finding the truth in psychological research is a challenge, and seemingly insurmountable difficulties are often encountered. Nonetheless, persistent and diligent efforts using strategies developed by generations of methodologists should lead to scientific advancement. (PsycINFO Database Record (c) 2019 APA, all rights reserved).