One of the characteristic features of spontaneous speech is the occurrence of disfluency phenomena. Due to the covert nature of the speech planning process, one can only rely on surface phenomena in looking for the causes of a given disfluency phenomenon. It is, however, an open issue whether the same surface phenomenon is invariably caused by the same functional disturbance or whether different disturbances necessarily lead to different types of disfluency. Wrong word access and its subtype, false start, may also have a number of different causes that may also underlie other surface phenomena. On the basis of a corpus-based study, the following such causes have been found: phonetic similarity, grammatical correspondence, semantic similarity, meaning condensation, confusion of idioms, as well as possible effects of the speech situation. These causes do not form distinct groups; they often occur together, reinforcing one another, and their results may be other types of errors (contamination, grammatical error, etc.) as well. Thus, very complex disturbances of function and implementation can be seen to underlie cases of wrong word access, ones that may also play a role in producing other disfluency phenomena.