The aim of the paper is to analyse compounding and compoundness in Slovak, Czech and Polish. Compoundness is defined as a property of a complex word (compound) having at least two components with lexical meaning. The essential distinction between composition and derivation (affixation) lies in the number of bases in the structure of a word, so it is inevitable that we analyse the notions of base and affix. Subsequently, the paper proposes several criteria on the basis of which the following constituents are defined: base (a free morpheme with lexical meaning, e.g. terapia in the Slovak compound ergoterapia ‘ergotherapy’), baseoid (combining form, a bound morpheme with lexical meaning, e.g. ergo- in Slovak compound ergoterapia ‘ergotherapy’), affixoid (diachronically a free morpheme, synchronically a bound morpheme that has acquired a more general derivational meaning, e.g. Slovak -násť in dva-násť ‘twelve’), affix (a bound morpheme with a general derivational meaning, e.g. Slovak -ista “osoba” ‘person’ in gitar-ista ‘guitar player’). The definition of these constituents makes it possible to establish a more precise borderline between compounding and derivation and introduce three types of compounds: proper compounds, semi-compounds and quasi-compounds.