Schizophrenia (SCZ) and bipolar disorder (BD) are severe psychiatric disorders that result from the complex interplay between genetic and environmental risk factors. Shared genetic risk between SCZ and BD is well-established, yet the extent to which they share environmental risk factors remains unclear. A longstanding topic of debate has been the nature and extent of interactions between genetic and environmental risk factors. In this talk, I will summarise the genetic and environmental contributions to SCZ and BD and share some of our findings from investigations in the Swedish population.