Extravagant male plumage traits in birds are a classic example of sexual selection. However we know very little about the units that selection is acting upon, the genes themselves – what are they and how are they influenced by sexual selection? In this study I focused on in the evolution and genetics of colouration the galliform birds. Several novel loci were used to create a well resolved phylogeny of this group. This was then used to investigate and reconstruct the evolution of sexual plumage dichromatism. Four pigmentation genes were sequenced in an array of galliform species. A measure of the rate of evolutionary change (dN/dS) at these loci was then compared between lineages with different strengths of sexual selection, using sexual dichromatism as the main index of sexual selection. I found evidence for sexual selection acting at the MC1R locus, in the form of a robust correlation between dN/dS and sexual plumage dichromatism that was not found at any of the other loci. I then went on to investigate the evolution and population genetics of MC1R in the grouse, focusing on the strongly dichromatic black grouse and the relatively monochromatic red grouse. I found some evidence for an adaptive change at this locus between these species. Finally I used a candidate gene approach to investigate the role of several genes in avian pigmentation using the Japanese quail (Coturnix japonica) as a model system. I found evidence that the avian agouti gene is involved in dorso-ventral pigmentation patterning and a regulatory mutation at this locus that produces a yellow phenotype. In addition point mutations at MC1R and TYRP1 were found to be responsible for producing pigmentation variants. I then compared the expression of several of these candidate genes in male and female common pheasants (Phasianus colchicus) and found lower TYRP1 expression in males. Knowledge of the genetic basis of secondary sexual traits and the action of sexual selection at this level could have important implications for our understanding of the process of sexual selection as a whole.