Background: Aphasia is an acquired language impairment following brain damage which affects some or all language modalities: expression and understanding of speech, reading and writing. Approximately one-third of people who have a stroke experience aphasia. Objectives: To assess the effectiveness of speech and language therapy (SLT) for aphasia following stroke.Search methods: We searched the Cochrane Stroke Group Trials Register (last searched April 2009), MEDLINE (1966 to April 2009) and CINAHL (1982 to April 2009). In an effort to identify further published, unpublished and ongoing trials we handsearched the International Journal of Language and Communication Disorders, searched reference lists of relevant articles and contacted other researchers and authors. Selection criteria: Randomised controlled trials comparing SLT versus no SLT, SLT versus social support or stimulation, and one SLT intervention versus another SLT intervention. SLT refers to a formal speech and language therapy intervention that aims to improve language and communication abilities and in turn levels of communicative activity and participation. Social support and stimulation refers to an intervention which provides social support or communication stimulation but does not include targeted therapeutic interventions. Direct comparisons of different SLT interventions refers to SLT interventions that differ in terms of duration, intensity, frequency or method of intervention or in the theoretical basis for the SLT approach. Data collection and analysis. Two review authors independently extracted the data and assessed the quality of included trials. We sought missing data from study investigators if necessary. Main results: We included 30 trials (41 paired comparisons) in the review: 14 subcomparisons (1064 participants) compared SLT with no SLT; six subcomparisons (279 participants) compared SLT with social support and stimulation; and 21 subcomparisons (732 participants) compared two approaches to SLT. In general, the trials randomised small numbers of participants across a range of characteristics (age, time since stroke and severity profiles), interventions and outcomes. Suitable statistical data were unavailable for several measures. Authors' conclusions: This review shows some indication of the effectiveness of SLT for people with aphasia following stroke. We also observed a consistency in the direction of results which favoured intensive SLT over conventional SLT, though significantly more people withdrew from intensive SLT than conventional SLT. SLT facilitated by a therapist-trained and supervised volunteer appears to be as effective as the provision of SLT by a professional. There was insufficient evidence to draw any conclusions in relation to the effectiveness of one SLT approach over another. Plain language summary: Speech and language therapy for aphasia following stroke. Language problems following a stroke are called aphasia (or dysphasia). About one-third of all people who experience stroke develop aphasia, which can affect one or more areas of communication (speaking, understanding spoken words, reading and writing). Speech and language therapists are involved in the assessment, diagnosis and treatment of aphasia at all stages of recovery, and work closely with the person with aphasia and their carers. There is no universally accepted treatment that can be applied to every person with aphasia. We identified 30 trials involving 1840 randomised participants that were suitable for inclusion in this review. Overall, the review shows evidence from randomised trials to suggest there may be a benefit from speech and language therapy but there was insufficient evidence to indicate the best approach to delivering speech and language therapy.