There has always been a debate around the issue of what it is that improves learning: the instruction itself or the media used for it (Clark 1983; Kozma 1994). It has also been said (Kulik and Kulik 1991; Andrewartha & Wilmot 2001) that computer mediated learning, as opposed to traditional instruction, positively influences the students' achievement. However, some researchers (Clark 1983; Schramm 1977; Wiley 2002) point out that it is not the use of media that improves learning but the strategies and the material developed for this particular kind of instruction. Online tuition is a trend which is being introduced by many educational institutions both as a core instructional mode, as in the case of pure on-line or blended-learning courses, or a as a complement to traditional face-to-face learning (Ko and Rossen 2010). Online asynchronous learning is implemented in order to attract students who desire to receive a quality education regardless of time zones, location and distance. Synchronous online learning is being used to increase interaction between students and teachers. In blended learning these two modalities, asynchronous and synchronous learning, are frequently combined to design full courses that promote meaningful learning. This paper will examine the benefits and difficulties of the combination of asynchronous and synchronous learning modes in blended learning and the theoretical and practical implications in the design of effective blended-learning materials for foreign language learning.