Illegal, unreported and unregulated (IUU) fishing is a recognised global problem that undermines the integrity of responsible fisheries management arrangements and results in lost value to coastal states. The first attempt at quantifying the value and volume of IUU fishing in tuna fisheries within the Pacific Islands region was undertaken in 2016 using data from 2010-2015 (MRAG Asia Pacific, 2016). That study estimated the total volume of product either harvested or transhipped involving IUU activity in Pacific tuna fisheries was 306,440t, with an ex-vessel value of $616.11m. Nevertheless, the authors noted that the data and information underlying many of the estimates were highly uncertain and that the outputs should be seen as a ‘first cut’. In order to assess changes in the nature and extent of IUU fishing since that time, this study was commissioned as part of the Global Environment Facility-funded Pacific Islands Oceanic Fisheries Management Project II (OFMP II) to undertake a ‘2020 update’ of the original estimates. Broadly, the aim was to undertake an ‘apples vs apples’ update of the original estimates, using a consistent methodology and taking into account the latest available information. The study period covered the years 2017-2019. Importantly, this preceded any COVID-19 related impacts on monitoring, control and surveillance (MCS) and IUU activity in the region. Broadly, we used a ‘bottom up’ approach to quantify IUU fishing activity across key IUU risks in four categories: (i) unlicensed/unauthorised fishing, (ii) misreporting, (iii) non-compliance with other license conditions (e.g. shark finning) and (iv) postharvest risks (e.g. illegal transhipping). ‘Best estimate’ and minimum/maximum range values were generated for each risk, taking into account the best available information. Monte Carlo simulation was then used to produce probabilistic estimates of IUU activity, taking into account probability distributions assigned within the minimum and maximum range values. Using this approach, estimates of IUU volume and value were developed for each of the three main fishing sectors – purse seine (PS), tropical longline (TLL) and southern longline (SLL) – and then aggregated to produce an overall estimate for Pacific Islands region tuna fisheries. The report sets out the outcomes from the analysis, as well as the main messages arising. The report also identifies priorities for future MCS development in the region across both purse seine and longline fisheries.