This chapter explores the phenomenon of affixal rivalry, namely, the competition between synonymous affixes, using comparative data and the lexical semantic framework (LSF), which is a lexicalist theory in the generativist tradition. The LSF’s major goal is to explain derivational polysemy, and it uses the conceptual tools of (i) feature, (ii) underspecification, and (iii) merge and coindexation to address this issue. Crucially, these tools are indispensable to explaining why languages often have more than one derivational affix for one derivational function, as we demonstrate using paradigms of nominalization and adjectivization in English and Japanese. We interpret the observation of the same mechanisms underlying different phenomena not as a coincidence but rather as a suggestion that derivational polysemy and affixal rivalry are in fact two sides of the same coin, constituting a tight-knit interdependence on each other. As a related topic, we discuss the issue of base selection and elucidate key factors to successfully respond to the challenges raised in language acquisition and language change.