Recent theories of phonology hold that phonotactic well-formedness may be gradient, with some legal structures being more well-formed than others. Linguistic and psycholinguistic research has demonstrated that "within" morphemes, speakers encode both categorical (*n/Onset) and gradient (st/Onset greater than sin/Onset) phonotactic restrictions. This dissertation investigated whether "heteromorphemic" sequences can be gradiently well-formed. In the first investigation, the effect of consonant similarity (the OCP) was investigated in tautomorphemic and heteromorphemic environments using an oral reading task. Subjects were slower to begin producing monomorphemic words containing similar/identical consonants (e.g., ""k"i"ck"", ""c"og") than words containing dissimilar consonants. Similar gradient effects were found heteromorphemically (e.g., ""d"imm"ed"") for four different suffixes ("-ed", "-s", "-ing", "-ness") and an effect was found in compounds where subjects were slower to begin uttering compounds that have a geminate (e.g., "pe"n" "n"ame") than compounds that do not. This indicates that not all heteromorphemic sequences equally easy to produce. The second investigation extends this result to focus specifically on the phonological grammar. In this investigation, the speech errors of a brain-damaged individual, WRG, are reported. WRG suffers from a lexical impairment that leaves grammatical processing intact. Despite the lexical locus of the damage, his errors are distinctly grammatical in nature, repairing phonologically dispreferred sequences through epenthesis. WRG's errors thus provide a "window" onto the functioning of an intact phonological grammar and provide an opportunity to investigate whether the grammar encodes gradient heteromorphemic constraints. WRG was asked to produce multimorphemic words containing legal heteromorphemic sequences of varying well-formedness (stress clash: "briskness", "aloofness" vs. no stress clash: "cleverness"; obstruent-obstruent coda clusters:" wal"ked"" vs. sonorant-obstruent clusters: "ba"nned""). WRG made insertion errors that improved the well-formedness (e.g., brisk"id"ness; walk.t"id") and made significantly more of these repairs on words containing the dispreferred structures. Crucially, these errors are shown to not be articulatory in nature, indicating that the errors arose at a phonological, pre-articulatory stage of processing. These results indicate that heteromorphemic sequences are subject to gradient phonotactic constraints in the phonological grammar. The "Activation-Conditioned Faithfulness Hypothesis", relating lexical activation to grammatical processing is proposed to account for how grammatical errors can result from a lexical impairment. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]