This book discusses the phonology of glides and nasals in Polish and English on the basis of spelling and pronunciation errors. The aim of this study is to show the relatedness between spelling and pronunciation errors and the phonological systems of both languages in Standard Generative Phonology and Optimality Theory. The rule-based phonology, where the intricacies of gliding and nasalization are resolved by the application of fully productive rules, gives a coherent analysis of the investigated data corpus. Similarly, the constraint-based phonology, where the solution is offered by the system of universal constraints, successfully accounts for the examined phenomena. However, for generalizations concerning the insertion of the back glide, Standard Optimality Theory does not provide conclusive results. This study proves that the introduction of a two-level evaluation, as envisaged by Derivational Optimality Theory, satisfactorily handles the ongoing changes.