Obesity and type 2 diabetes (T2D) are major risk factors for cardiovascular and other diseases and are currently undergoing an increase in global prevalence. The work presented in my thesis addresses the role epigenetics, specifically DNA methylation, plays in the susceptibility to obesity and T2D and deals with methodological issues in the analysis of DNA methylation data. I first combined epigenome-wide DNA methylation data across 38 adipose tissue samples with corresponding SNP and mRNA data for the same subjects. At 5% false discovery rate (FDR), methylation of 149 regions associated with at least one cis-SNP. When 19 of the 149 regions were tested for association in an additional 181 independent samples, five regions replicated. These results indicate a genetic influence on DNA methylation in adipose tissue. I then analysed 90 epigenome-wide methylation samples taken from 15 South Asian controls and 30 T2D cases participating in the LOLIPOP study at two time points ∼7 years apart. I found global differences at both follow-up and baseline between the normal glucose tolerant and T2D groups, as well as strong differences with aging. I further used the main EpiMigrant data from 2,687 individuals, with 36 samples measured in duplicate to assess approaches to quality control, data normalisation and batch correction through control probe adjustment. A null hypothesis for epigenome-wide association studies (EWAS) by permutation testing and I investigated the effects of correlation between individual methylation markers. Using the developed methods, I carried out an EWAS of body mass index (BMI) with subsequent meta-analysis amongst 10,261 individuals of European and South Asian ancestry. DNA methylation markers at 187 genetic loci were associated with BMI. Mendelian randomisation experiments suggested that association of DNA methylation with BMI is the consequence of BMI. Lastly, I tested haplotypes of 85 SNPs currently known to be associated with T2D and 118 SNPs associated with obesity traits for an enrichment of CpG creating or abrogating SNPs and found that 9 T2D and 23 obesity SNPs showed a significant difference in CpG count between the SNP alleles as established by permutation testing. Amongst these is FTO, a locus which has been previously been shown to have a haplotype-specific methylation effect. My work provides novel insights into the role of DNA methylation in metabolic diseases. The methods that I developed to robustly detect association are flexible and scalable and will further be useful for larger, future EWAS.