Research on the internationalisation of business firms has investigated the decision to export and the volume of sales abroad, independently of the decision about the exporting mode. This paper presents a unified theoretical framework to integrate the two decisions, which departs from imperfect competition, sunk costs and transaction costs. The model provides an explanation to the empirical evidence on the incremental stages of internationalisation followed by many firms. The main hypothesis is tested with data from Spanish exporters and the results confirm that asset specificity is the underlying factor that justifies the degree of internationalisation, specially for small and medium size firms.