In this contribution we discuss diachronic and variationist aspects of morphonotactics, a new research field that we have tried to establish over the last years (cf. Dressler & Dziubalska-Kołaczyk, Rivista di Linguistica, 18: 249–266, 2006). Morphonotactics is the area of interaction between morphotactics and phonotactics and represents a subfield of morphonology, which in turn is the area of interaction between morphology and phonology (cf. Dressler, Morphonology, Karoma Press, 1985, A functionalist semiotic model of morphonology, Benjamins, 1996). We claim that in this interaction morphotactics typically creates phonotactically marked structures which occur never or only exceptionally in monomorphemic words. In our contribution we deal with typical diachronic changes. Our claim about the markedness of morphonotactic sequences is tested mainly against data from Polish, Lithuanian and other Balto-Slavic languages. Our theoretical basis draws on models of Natural Phonology (cf. Hurch & Rhodes, Natural Phonology: The state of the art, Mouton de Gruyter, 1996, Dziubalska-Kołaczyk & Weckwerth, Future challenges for natural linguistics, Lincom, 2002) and Natural Morphology (cf. Dressler et al., Leitmotifs in natural morphology, Benjamins, 1987, Kilani-Schoch & Dressler, Morphologie naturelle et flexion du verbe françis, Narr, 2005), and especially on the subtheories of universal markedness (or universal preferences) and of typological adequacy.