One purpose of empirical software engineering is to enable an understanding of factors that influence software development. Surveys are an appropriate empirical strategy to gather data from a large population (e.g., about methods, tools, developers, companies) and to achieve an understanding of that population. Although surveys are quite often performed, for example, in social sciences and marketing research, they are underrepresented in empirical software engineering research, which most often uses controlled experiments and case studies. Consequently, also the methodological support how to perform such studies in software engineering is rather low. However, with the increasing pervasion of the Internet it is possible to perform surveys easily and cost-effectively over Internet pages (i.e., on-line), while at the same time the interest in performing surveys is growing. The purpose of this paper is twofold. First we want to arise the awareness of on-line surveys and discuss methods how to perform these in the context of software engineering. Second, we report our experience in performing on-line surveys in the form of lessons learned and guidelines.