As Cheng（2003）commented，the study on coherence is far less prosperous and fruitful than that on cohesion.The breakthrough in the field of cohesion studies is the cohesion theory by Halliday and Hasan（1976）.It has been discussed，commented，applied，and modified by numerous scholars in various ways（see for example，Carrell，1982;Hu，1994;Zhang，2001;Cheng，2003;Zhu and Yan，2001）.

Although the concept of cohesion is intended to be a semantic one in Halliday and Hasan（1976），more researchers tend to regard it as the formal cues or signals of the coherence relations in the text.Cohesive devices“operate on the superficial surface structure of a text”（Carrell，1982），and should be regarded as something at the surface level of coherence.It is the semantic meaning，rather than cohesive devices，that lies at the heart of coherence.For example，according to Taboada（2006），the percentage of the coherence relations which are explicitly signaled is low，that is，31%of the time in conversation and 44%in newspaper articles.The results prove that the coherence relations can be realized as explicit ones and implicit ones，that is，some coherence relations are explicitly signaled whereas others are not.

On the other hand，cohesive devices are also important in the establishment of coherence in that they have a facilitative role in constructing coherence.They can facilitate readers to quickly recognize and grasp the relations between elements and between parts and whole，and thus help readers to construct a coherence mental representation of the text.As Halliday and Hasan（1976）and many other researchers have shown，the analysis on cohesive devices in texts can provide a method to reveal the weaving of texture and have great significance to the study on coherence.

In Systemic Functional Linguistics，Halliday and Hasan（1985）further extend the range of cohesion.Hu（1994）proposes a multi-layered framework of discourse cohesion and coherence.Zhang and Liu（2003）proposes that cohesion should also include the external contextual factors.In these frameworks the concept of cohesion is often used together with that of coherence，which is a feature of Systemic Functional Linguistics.In other words，their frameworks are addressed to“coherence and cohesion”as a single，inseparable concept rather than to coherence or to cohesion.

Different from the systemic functional linguists，most researchers hold the view that coherence should subsume cohesion，and cohesion is the formal linguistic markers of coherence.In this regard，the taxonomy of cohesion in Halliday and Hasan（1976）is still the blueprint of the frameworks of studying cohesive devices in various approaches.The relationship between cohesion and coherence is well summarized in Sanders and Spooren（2009）：

...coherence may，but need not，be signaled explicitly in the text.This is why we advocate a coherence approach：the connectedness is a characteristic of the mental representation of the discourse rather than of the discourse itself.Although coherence phenomena are of a conceptual nature，their reconstruction is often based on linguistic signals in the discourse itself.These linguistic expressions are considered processing instructions to language users...

The cohesive devices in their view include connectives（because，however）and（other）lexical markers of coherence relations，such as cue phrases（on the one hand...on the other hand）and signaling phrases（the problem is...，a solution might be...）that make the meaning relations between discourse segments explicit.

To summarize，“the relation between the linguistic surface code on the one hand（so-called cohesion phenomena）and aspects of discourse reprehension on the other hand（coherence），has become a crucial research issue in the interdisciplinary field of discourse studies”（Sanders and Spooren，2009：198）.