This thesis examines the influence of some characteristics of the agreement target on the performances of novice and expert writers in number agreement via different tasks. A first series of experiments studies the impact of the presence of morpho-phonological and conceptual information carried by the verb. Results show that from the 5th grade, aphonologically distinct verbal ending between singular and plural improves agreement processing. The presence of a conceptual information on the verb leads however participants from the 4th grade to commit more agreement errors. The processing of a verb whose action is necessarily accomplished by several agents seems to create disturbances during theagreement. Morpho- phonological and conceptual information also influence the duration of production of dictated sentences and the number of agreement errors correctly detected in revision tasks. A second series of experiments analyzes the impact of cognitive cost incurred by the complexity of the management of lexical spelling on the treatment of agreement, by manipulating lexical frequency and orthographic consistency. The effect of these variables on the errors committed by children suggests that the probability of agreement in number varies with frequency (lexical factor) and consistency (sub- lexical factor) and this, from 2nd to 5th grade. Overall, the results of this thesis suggest that agreement is not processed independently from some specific characteristics of the agreement's target
