The studies on coherence have traditionally been conducted on the basis of purely linguistic texts rather than multimodal texts.Even when only one semiotic mode—language—is concerned，coherence is a highly controversial term.As many linguists（see for example，Cheng，2003;Bednarek，2005）observe，coherence is a rather fuzzy notion in linguistics and there has not been a generally accepted definition or theoretical framework of coherence yet（Zhang，1999）.There are various definitions and interpretations of coherence although all of them are concerned with the hanging-together of the text.

It's widely acknowledged that coherence involves different levels.For example，Samet and Schank（1984）draw a distinction between two different sorts of coherence.The first involves the internal connectivity of the hearer's representation of the text，that is，the connections between smaller units glue together the representations of the parts into a unified representation of the whole.The second sort of connectivity concerns the integratability of this unified representation into the hearer's existing conceptual framework which is external in that it holds between the hearer's representation of present input and his world knowledge stored in his long-term memory.The first sort of connectivity，that is，the internal connectivity，can be further classified into local coherence and global coherence^[1].The expository connections between sentences，such as examples，generalizations，paraphrases，contrasts，etc.，are ways of creating local coherence.The gist of narrative—the story line，plot，or thread of the narrative，on the other hand，governs the global coherence and its main purpose is to help eliminate locally coherent nonsense texts.

The distinction between internal and external and that between local and global has been supported in many other works（see for example，Hu，1994;Givón，1995;van Dijk and Kintsch，1983;Giora，1985;Zhang，1999;Zhang and Liu，2003;Zhu and Yan，2001）.Therefore，under the text-internal factors of coherence，there is a three-way division among the various studies：those on coherence at a more global scope of texts，those at the local scope of texts，that is，the relations between sentences or clause complexes，and those on the explicit cohesive devices in the expression level of texts.

This section will first review in Sub-section 2.1.1 the definitions of coherence，often in comparison to cohesion，followed by a review of the different levels of discourse coherence which will follow the distinctions mentioned above.Specifically，external factors of coherence will be reviewed first in Sub-section 2.1.2.An important distinction in the text-internal aspects of coherence—the one between global coherence and local coherence—will be reviewed in Sub-section 2.1.3 and Sub-section 2.1.4 respectively.Apart from these levels of coherence，cohesion as another very important factor in the weaving of texture through the operation at the surface level will be reviewed in Subsection 2.1.5.