Knowing words involves knowing multiple aspects of word knowledge (Nation, 2013). Most previous research focused on single aspects and measured the ultimate gains at single points of time. The uniqueness of this dissertation lies in detecting and comparing the productive incre-mental changes of three aspects of word knowledge (meaning, spelling, and word part) in Educa-tional (e.g. school) and naturalistic settings (e.g. workplace) at different time intervals over a 24-month period. Using Nation's (2013) Word Knowledge Framework, two main studies and a follow-up study were conducted from January 2018 to January 2020. Five upper-intermediate Arabic speaking learners participated in Study 1, and two advanced Arabic speaking learners participat-ed in Study 2. Three participants from Study 1 and both participants From Study 2 participated in the Follow-up Study. The same battery of pen-and-paper tests (i.e., spelling, multiple choice and fill-in-the-blanks) was used to elicit quantitative data from participants in both the two main and the fol-low-up studies, which were then analyzed statistically. Qualitative data, drawn from semi-structured interviews and classroom observations were analyzed using Saldaña's (2013) first cy-cle and top down coding method. Results countered those previously reported (e.g., González-Fernández & Schmitt, 2019; Schmitt, 1998). A developmental hierarchy was detected among the three measured aspects of word knowledge. They developed concurrently and in varying proportions. Basic meaning knowledge always enjoyed the highest gains followed by spelling knowledge and word part knowledge. A relationship between the incremental changes of the three measured aspects of word knowledge and participants' vocabulary size was also detected. Several vocabulary learning strategies including (but not limited to) word lists, word parts, and orthographic repetition seemed to play a positive role in word knowledge develop-ment. Several factors such as lack of adequate word exposure opportunities as well as learners' first language (L1) transfer were found to negatively affect word knowledge development. This study concludes that certain aspects require more attention and time than others. Learners need to be exposed several times to the target words and in different contexts. They need to be taught and trained to use the different vocabulary learning strategies to enhance attainment.