Supporters of languages planned for international communication, like Esperanto, often claim that these languages are less complex and therefore easy to learn as compared to natural languages. To what extent does this claim have empirical support? In this contribution, planned languages will be presented from the perspective of learnability. In particular, the question of language complexity will be addressed. Almost all planned languages show a high degree of morphological regularity, obtained by a drastic reduction of allomorphy and suppletion. While these morphological traits can help learners acquire the basics of the planned language more easily as compared to standard natural languages, other factors should be taken into account in order to assess the learnability of these languages, in particular their sociolinguistic status.