The goal of this thesis is to add a typologically distinct data point to the investigation of access and representation of words in the mental lexicon, which until recently has been biased towards English. We concentrated on Polish, which contrasts with English in the richness of its inflectional and derivational morphology and its morpho-phonological alternations. Using immediate cross-modal and delayed auditory-auditory priming, parallel issues to those examined in English are investigated, as well as questions which cannot be addressed in English, because of differences in morphological properties. Four main findings are reported. First, the representation of morphologically complex Polish words is combinatorial, similar to English; This is supported by: (a) robust priming for items which share the same stem; (b) affix priming for morphologically complex items; (c) suffix-suffix interference for items competing for the same stem; Second, the results on Polish highly and moderately semantically transparent compounds suggest that the former may be represented in a combinatorial format and the latter as full forms. This contrasts with English where both types of compound are claimed to be stored as full forms. Third, initial investigations of the role of semantic transparency in determining how morphologically complex words are represented, suggest that transparent items are stored decompositionally whereas opaque items are stored as full forms. This conforms to the English findings, but contrasts with Semitic languages. Further investigations indicate that semantic compositionality may be more important that transparency, although more research is needed here. Finally, the results suggest that phonological alternants of the same stem, whether regular or irregular, are stored in a single lexical entry, and, in the auditory modality, map directly onto the same abstract underlying representation. Overall the results support the claim that the Polish lexicon IS organised morphemically in a combinatorial, phonologically abstract format