Abstract Notwithstanding certain similarities, Belgium and the Netherlands have different national histories. Keeping this in mind, this chapter is divided into four sections: early history, pillarization, secularization, and Islam and new developments. From its foundation in 1830, Belgium has been predominantly Catholic, whereas the Netherlands claimed to be a Protestant nation, despite a large minority of Catholics. In the late nineteenth century, self-contained worlds (‘pillars’) emerged in both countries. Catholics, and in the Netherlands orthodox Protestants as well, used their many-branched pillars of societal organizations to emancipate and mobilize their constituencies. In the 1960s, the pillars started to crumble and the number of non-affiliated rose to 42 per cent in Belgium, and 68 per cent in the Netherlands. Notwithstanding the immigration of significant groups of Christians and Muslims and a flourishing market in spirituality, both countries have become very secularized. A final note summarizes the situation in Luxembourg.