The Duvergerian theories have largely been the frame of reference upon which parties’ and voters’ strategic decisions have been explained. In this chapter I briefly summarise this literature, I identify the determinants of party entry decisions in Duvergerian contexts and finally I explain how the overlap of electoral arenas can undermine this logic. The chapter is organised as follows: Section 2.1 summarises the Duvergerian theories, whereas section 2.2 deals with the benefits and the costs of competing and summarises how party-entry decisions are expected to be taken under the Duvergerian assumptions. Section 2.3 argues that the assumption of the independence of electoral arenas that the literature has purported up to now is false, thus enabling the emergence of contamination effects between different arenas. This section proposes a new and more encompassing definition of the phenomenon of contamination effects and its theoretical implications. Section 2.4 summarises the main arguments of the Chapter.