Background: The reading skills of 16% of children fall below the mean range for their age, and 5% of children have significant and severe reading problems. Phonics training is one of the most common reading treatments used with poor readers, particularly children. Objectives: To measure the effect of phonics training and explore the impact of various factors, such as training duration and training group size, that might moderate the effect of phonics training on literacy-related skills in English-speaking poor readers. Search methods: We searched CENTRAL, MEDLINE, Embase, 12 other databases, and three trials registers up to May 2018. We also searched reference lists of included studies and contacted experts in the field to identify additional studies. Selection criteria: We included studies that used randomisation, quasi-randomisation, or minimisation to allocate participants to a phonics intervention group (phonics training only or phonics training plus one other literacy-related skill) or a control group (no training or non-literacy training). Participants were English-speaking poor readers with word reading one standard deviation below the appropriate level for their age (children, adolescents, and adults) or one grade or year below the appropriate level (children only), for no known reason. Participants had no known comorbid developmental disorder, or physical, neurological, or emotional problem. Data collection and analysis: We used standard methodological procedures expected by Cochrane. Main results: We included 14 studies with 923 participants in this review. Studies took place in Australia, Canada, the UK, and the USA. Six of the 14 included studies were funded by government agencies and one was funded by a university grant. The rest were funded by charitable foundations or trusts. Each study compared phonics training alone, or in conjunction with one other reading-related skill, to either no training (i.e. treatment as usual) or alterative training (e.g. maths). Participants were English-speaking children or adolescents, of low and middle socioeconomic status, whose reading was one year, one grade, or one standard deviation below the level expected for their age or grade for no known reason. Phonics training varied between studies in intensity (up to four hours per week), duration (up to seven months), training group size (individual and small groups), and delivery (human and computer). We measured the effect of phonics training on seven primary outcomes (mixed/regular word reading accuracy, non-word reading accuracy, irregular word reading accuracy, mixed/regular word reading fluency, non-word reading fluency, reading comprehension, and spelling). We judged all studies to be at low risk of bias for most risk criteria, and used the GRADE approach to assess the quality of the evidence. There was low-quality evidence that phonics training may have improved poor readers' accuracy for reading real and novel words that follow the letter-sound rules (standardised mean difference (SMD) 0.51, 95% confidence interval (CI) 0.13 to 0.90; 11 studies, 701 participants), and their accuracy for reading words that did not follow these rules (SMD 0.67, 95% CI 0.26 to 1.07; 10 studies, 682 participants). There was moderate-quality evidence that phonics training probably improved English-speaking poor readers' fluency for reading words that followed the letter-sounds rules (SMD 0.45, 95% CI 0.19 to 0.72; 4 studies, 224 participants), and non-word reading fluency (SMD 0.39, 95% CI 0.10 to 0.68; 3 studies, 188 participants), as well as their accuracy for reading words that did not follow these rules (SMD 0.84, 95% CI 0.30 to 1.39; 4 studies, 294 participants). In addition, there was low-quality evidence that phonics training may have improved poor readers' spelling (SMD 0.47, 95% CI -0.07 to 1.01; 3 studies, 158 participants), but only slightly improve their reading comprehension (SMD 0.28, 95% CI -0.07 to 0.62; 5 studies, 343 participants). Authors' conclusions: Phonics training appears to be effective for improving literacy-related skills, particularly reading fluency of words and non-words, and accuracy of reading irregular words. More studies are needed to improve the precision of outcomes, including word and non-word reading accuracy, reading comprehension, spelling, letter-sound knowledge, and phonological output. More data are also needed to determine if phonics training in English-speaking poor readers is moderated by factors such as training type, intensity, duration, group size, or administrator.