Background: Children with Down syndrome have speech and language difficulties that are disproportionate to their overall intellectual ability and relative strengths in the use of gesture. Shared book reading between parents and their children provides an effective context in which language development can be facilitated. However, children with Down syndrome often take a passive role in shared book reading and the use of key word signing (KWS) as a shared book reading technique has never been investigated. Aims: This study aimed to compare children with Down syndrome's participation and use of KWS across two methods of shared book reading -- one in which a book had key-word sign prompts embedded (signed condition) and the other in which a book was read as normal (unsigned condition). Measures of child and parent communicative behaviour were taken in each condition to establish if differences emerged. Methods & Procedures: A total of 36 children with Down syndrome (aged between 18 and 61 months) and their mothers took part in the study. Parent-child dyads were videoed at home reading two books, one in a signed and one in an unsigned condition. Child measures included total number of signs produced in each condition and levels of attention and initiation as measured by the Pivotal Behaviour Rating Scale. Parent measures included total number of utterances, mean length of utterance (MLU) in morphemes and vocabulary diversity (VOCD). Parental measures were transcribed using the Codes for Human Analysis Transcripts (CHAT) software and analysed by the Computerised Language Analysis software (CLAN). Contrasts in outcomes between the signed and unsigned conditions were estimated using Poisson and linear mixed-effects models, determined by the type of data. Outcomes & Results: Results showed that children attempted to sign significantly more in the signed than unsigned condition, as well as showing significant increases in their levels of attention and initiation. There was also a significant increase in the total number of utterances used by parents in the signed versus unsigned condition and a decrease in MLU. VOCD was similar in both conditions. Conclusions & Implications: This study shows that the simple act of embedding key word signs into commercially available books, during shared book reading between parents and young children with Down syndrome, positively affects children's participation (initiation and attention) and use of KWS. The use of KWS as a core shared book reading technique may therefore be a fruitful avenue to facilitate growth in the language abilities of young children with Down syndrome.