Obesity is a highly heterogenous condition with a complex biology and an estimated heritability of 40-70%. Studies of obesity genetics have expanded our biological understanding of obesity showing that obesity may be due to a single rare mutation with large effect size or the sum of multiple genetic variants each with small effect sizes. In this review, we present current knowledge of obesity genetics covering aspects related to age and ethnicity. We present how the technology has moved us beyond BMI into obesity-related traits, adipocyte biology, and biomarkers aiding future clinical practice.