Living systems are intrinsically noisy. Surprisingly, the functioning of a living organism is not significantly hindered by these random fluctuations. Biological cells can even exploit noise by deliberately introducing diversity into a population. In these cases noise is not a nuisance, but essential for survival. Advances in modern biochemistry and genetics have led to a detailed understanding of the molecular machinery involved in gene expression, and the constant flow of data from the Genome Project has enabled the identification of more and more genes. A millennial challenge is to quantitatively understand how different genes and their regulating proteins are grouped together in genetic circuits, and how stochastic fluctuations influence gene expression in these complex systems. In our group we focus on the importance of noise in the expression of genes by using both experimental and theoretical approaches.