Background: Communication and language development are areas of particular weakness for young children with Down syndrome. Caregivers' interaction with children influences language development, so many early interventions involve training parents how best to respond to their children and provide appropriate language stimulation. Thus, these interventions are mediated through parents, who in turn are trained and coached in the implementation of interventions by clinicians. As the interventions involve a considerable commitment from clinicians and families, we undertook this review to synthesise the evidence of their effectiveness. Objectives To assess the effects of parent‐mediated interventions for improving communication and language development in young children with Down syndrome. Other outcomes are parental behaviour and responsivity, parental stress and satisfaction, and children's non‐verbal means of communicating, socialisation and behaviour. Search methods In January 2018 we searched CENTRAL, MEDLINE, Embase and 14 other databases. We also searched three trials registers, checked the reference lists of relevant reports identified by the electronic searches, searched the websites of professional organizations, and contacted their staff and other researchers working in the field to identify other relevant published, unpublished and ongoing studies. Selection criteria We included randomised controlled trials (RCTs) and quasi‐RCTs that compared parent‐mediated interventions designed to improve communication and language versus teaching/treatment as usual (TAU) or no treatment or delayed (wait‐listed) treatment, in children with Down syndrome aged between birth and six years. We included studies delivering the parent‐mediated intervention in conjunction with a clinician‐mediated intervention, as long as the intervention group was the only group to receive the former and both groups received the latter. Data collection and analysis We used standard Cochrane methodological procedures for data collection and analysis. Main results We included three studies involving 45 children aged between 29 months and six years with Down syndrome. Two studies compared parent‐mediated interventions versus TAU; the third compared a parent‐mediated plus clinician‐mediated intervention versus a clinician‐mediated intervention alone. Treatment duration varied from 12 weeks to six months. One study provided nine group sessions and four individualised home‐based sessions over a 13‐week period. Another study provided weekly, individual clinic‐based or home‐based sessions lasting 1.5 to 2 hours, over a six‐month period. The third study provided one 2‐ to 3‐hour group session followed by bi‐weekly, individual clinic‐based sessions plus once‐weekly home‐based sessions for 12 weeks. Because of the different study designs and outcome measures used, we were unable to conduct a meta‐analysis. We judged all three studies to be at high risk of bias in relation to blinding of participants (not possible due to the nature of the intervention) and blinding of outcome assessors, and at an unclear risk of bias for allocation concealment. We judged one study to be at unclear risk of selection bias, as authors did not report the methods used to generate the random sequence; at high risk of reporting bias, as they did not report on one assessed outcome; and at high risk of detection bias, as the control group had a cointervention and only parents in the intervention group were made aware of the target words for their children. The sample sizes of each included study were very small, meaning that they are unlikely to be representative of the target population. The findings from the three included studies were inconsistent. Two studies found no differences in expressive or receptive language abilities between the groups, whether measured by direct assessment or parent reports. However, they did find that children in the intervention group could use more targeted vocabulary items or utterances with la guage targets in certain contexts postintervention, compared to those in the control group; this was not maintained 12 months later. The third study found gains for the intervention group on total‐language measures immediately postintervention. One study did not find any differences in parental stress scores between the groups at any time point up to 12 months postintervention. All three studies noted differences in most measures of how the parents talked to and interacted with their children postintervention, and in one study most strategies were maintained in the intervention group at 12 months postintervention. No study reported evidence of language attrition following the intervention in either group, while one study found positive outcomes on children's socialisation skills in the intervention group. One study looked at adherence to the treatment through attendance data, finding that mothers in the intervention group attended seven out of nine group sessions and were present for four home visits. No study measured parental use of the strategies outside of the intervention sessions. A grant from the Hospital for Sick Children Foundation (Toronto, Ontario, Canada) funded one study. Another received partial funding from the National Institute of Child Health and Human Development and the Department of Education in the USA. The remaining study did not specify any funding sources. In light of the serious limitations in methodology, and the small number of studies included, we considered the overall quality of the evidence, as assessed by GRADE, to be very low. This means that we have very little confidence in the results, and further research is very likely to have an important impact on our confidence in the estimate of treatment effect. Authors' conclusions There is currently insufficient evidence to determine the effects of parent‐mediated interventions for improving the language and communication of children with Down syndrome. We found only three small studies of very low quality. This review highlights the need for well‐designed studies, including RCTs, to evaluate the effectiveness of parent‐mediated interventions. Trials should use valid, reliable and similar measures of language development, and they should include measures of secondary outcomes more distal to the intervention, such as family well‐being. Treatment fidelity, in particular parental dosage of the intervention outside of prescribed sessions, also needs to be documented. Plain language summary Parent‐mediated interventions to promote communication and language development in young children with Down syndrome Review question Do parent‐mediated interventions improve communication and language development in young children with Down syndrome? Background Language development is an area of particular weakness for young children with Down syndrome. Caregivers' interaction with children influences language development, so sometimes clinicians coach parents so they can stimulate their children's language and communication skills. Study characteristics The evidence is current to January 2018. We found three studies involving 45 children aged between 29 months and six years. Two studies were randomised controlled trials: experiments in which children were allocated to treatment (i.e. parent‐mediated) and control (treatment as usual or clinician‐mediated, or both) groups using a random method such as a computer‐generated list of random numbers. The other study reported that randomisation took place but did not specify how this was done. Two studies compared parent‐mediated intervention to treatment as usual. One of these lasted for 13 weeks, and parents in the intervention group received nine, weekly group sessions and four individual sessions in the home. The total intervention time was approximately 26.5 hours. A second study lasted for six months, and parents received weekly, 1.5‐ to 2‐hour clinic or home‐based, individualised, parent‐child sessions. The total intervention time was approximately 48 hours. A third study compared parent‐ and clinician‐mediated intervention to a clinician‐only‐mediated intervention. In this study the parents in the intervention group took part in a two‐ to three‐hour interactive workshop plus three individualised sessions (two clinic‐based and one home‐based) every week for 12 weeks. The control group received the same individualised sessions, but a clinician delivered them (i.e. there was no parental involvement). The total intervention time was approximately 19 hours. A grant from the Hospital for Sick Children Foundation (Toronto, Ontario, Canada) funded one study. Another received partial funding from the National Institute of Child Health and Human Development and the Department of Education in the USA. The remaining study did not specify any funding sources. Key results Two of the three studies found no differences in children's language ability after parent training. However, these same two studies found that children in the intervention group used more words that had been specifically targeted, postintervention; this was not maintained 12 months later. The study that gave parents the largest amount of intervention reported gains on general measures of overall language ability for children in the intervention group. One study did not find any changes in levels of parental stress immediately or up to 12 months postintervention in either group. All three studies noted changes in how parents talked to and interacted with their children immediately postintervention, and most strategies were retained by the intervention group 12 months later. One study reported increases in the socialisation skills of children who received the intervention. No study reported language attrition in either group postintervention. Quality of the evidence We rated the quality of the evidence in this review as very low, as only three studies fulfilled the criteria for inclusion, and all had small sizes and serious methodological limitations. There is currently insufficient evidence to determine the effect of parent‐mediated interventions for improving the communication and language development in young children with Down syndrome.