Native-English speaking adults use morphological decomposition to understand complex words (e.g. "farmer" becomes "farm-er"). Whether decomposition is driven by semantic organization is still unclear. It is also unclear whether ESL adults and elementary age children use the same word processing strategies as native speaking adults. This study tested an identical experimental procedure across three English-speaking populations: native speaking adults, non-native speaking adults and elementary age children. The first task tested how readers use base and suffix information in complex words and nonwords when the word featured only a base word, only a suffix, both a base and a suffix or neither. The second task was a masked priming task that evaluated how fast readers processed a word when paired with a transparent (farmer), opaque (corner) or simple (castle) word prime. For the first task, results showed both native and nonnative adult English speakers use base and suffix information for English words and nonwords and vocabulary proficiency influences native speaker English word accuracy and non-native speaker nonword accuracy. Elementary age children did not use base or suffix information consistently for English words but used base word information for complex nonwords. Results for the second task, masked priming, showed a significant positive priming effect for transparent word pairs in all three language groups, significant positive priming in native speaking adults for opaque words and significant positive priming in children for simple words. Results for native speakers suggest a morphological decomposition strategy is obligatory. While non-native speakers of English use both base and suffix information when reading, morphological decomposition for this group is not obligatory. For the non-native speakers, age of acquisition did not interact with any of the experimental variables. Both tasks show elementary age children are still learning the rules of morphological decomposition and learning when morphological decomposition is an efficient strategy. The results of these studies have implications for vocabulary and literacy curriculum in both ESL and developing reader classrooms. Both learner groups would benefit from explicit suffix decomposition instruction as well as instruction regarding the semantic and grammatical role suffixes serve in word formation. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]