In the current talk, I review experimental and observational work on aspects of the evolution of polyandry in free-living sand lizards (Lacerta agilis) in southern Sweden. This work demonstrates polyandry effects on the probability of fertility at the onset of the mating season in a simulated ancestral population (with simultaneous male-female emergence, as opposed to [the current] protandric male emergence from hibernation). However, when fertility was assessed fertility in the current population, polyandry had no effect. Thus, assuming that polyandry is a costly behaviour (e.g., due increased predation, risk of sexually transmitted disease etc) something else must explain its persistence in the population. Lab experiments at the beginning of this research project showed that polyandry not only increased offspring viability at hatching but also offspring first-year survival (when mortality is at its highest). Since sand lizards are ectotherms, we then looked for environmental effects on adult courtship and mating behaviour and could confirm that during more benign conditions (warmer mating seasons) males were more active courting females, which resulted in more copulations per female and year, higher degree of multiple paternity within clutches, less malformations in the offspring, and higher offspring first year survival. Thus, we could confirm that our first lab experiments verifying positive polyandry effects on offspring viability were congruent with our observations on corresponding viability effects over a decade in the wild, comprising more than 5,00 adult and 4,000 offspring.