Study 1 collected 263 effect sizes from 22 studies reporting the correlation coefficients between listener-based judgements of fluency and objective measures of temporal features (N = 335–746). Finally, Study 4 investigated the extent to which L2 utterance fluency can be predicted from L1 utterance fluency with regard to the moderator effects of L2 proficiency on the L1-L2 utterance fluency link. The study also analysed the stability of the factor structure of utterance fluency (Tavakoli & Skehan, 2005)-speed, breakdown, and repair fluency-and cognitive fluency across speaking tasks. Study 3 examined the contribution of cognitive fluency to utterance fluency, taking a structural equation modelling (SEM) approach. Study 2 compared utterance fluency performance across speaking tasks which were designed to differ in the quality of speech processing demands, operationalized by task design features (i.e., task effects). Study 1 took a meta-analytic approach to synthesizing previous findings on the relationship between perceived and utterance fluency. To this end, this thesis consists of four separate studies. The overarching goal of the thesis is to examine the construct of L2 oral fluency, particularly focusing on the interrelationship between cognitive, utterance, and perceived fluency. However, it is still unclear how these three subconstructs of L2 fluency are interrelated with each other. In order to distinguish different conceptualizations of fluency, Segalowitz (2010) proposed three subconstructs of fluency: utterance fluency (i.e., observable temporal features of speech), cognitive fluency (i.e., speaker’s ability to manipulate L2 knowledge efficiently), and perceived fluency (i.e., listener’s subjective judgements of fluency). However, L2 fluency research has witnessed a long debate over the definition and measurements of L2 oral fluency scholars have interchangeably used the term, “fluency”, with different connotations, such as speakers’ ability, speech features, and listeners’ perception. In the context of the learning, teaching, and assessment of second language (L2) speaking skills, L2 fluency has been regarded as one of the important constructs.