Wide-ranging research interests span all areas of systematic review and meta-analysis. Julian has co-authored more than 100 applied systematic reviews and over 200 papers on evidence synthesis methodology. Among his methods contributions are: a Bayesian approach to network meta-analysis; the I-squared statistic to quantify inconsistency across studies in a meta-analysis; simple prediction intervals for random-effects meta-analysis; a general framework for individual participant data meta-analysis; a library of prior distributions for between-study variation in a meta-analysis; and risk-of-bias assessment tools for clinical trials and other study designs. He was a member of the development teams for the original PRISMA reporting guideline for systematic reviews and meta-analyses and the STROBE-MR reporting guidelines for Mendelian randomization studies, and co-led development of the STREGA reporting guideline for genetic association studies.

Julian was a founding trustee and is a past President of the Society for Research Synthesis Methodology. He has long been an active contributor to Cochrane, and is a former member of Cochrane's Steering Group, Editorial Board and Scientific Committee. He is currently co-convenor of the Cochrane Bias Methods Group and has co-edited the Cochrane Handbook for Systematic Reviews of Interventions since 2003. He is also co-author of the Wiley 2009 textbook Introduction to Meta-analysis and co-editor of the 2022 3rd edition of Wiley textbook Systematic Reviews in Health Research: Meta-analysis in Context. He has more than 350 publications, which have attracted more than 700,000 citations. With a Google Scholar H-index over 200, Julian has been named as a Highly Cited Researcher each year since 2015.

Julian is a multiple winner of the Thomas C Chalmers Award from Cochrane. He was awarded the Frederick Mosteller Award for distinctive contributions to systematic reviewing from the Campbell Collaboration in 2010; the Ingram Olkin Award for distinguished lifetime achievement by the Society for Research Synthesis Methodology in 2016; and the Will Shadish Extraordinary Service Award by the Society for Research Synthesis Methodology in 2019. He was appointed an NIHR Senior Investigator in 2018 and an NIHR Emeritus Senior Investigator in 2026.