Numerous land-based studies have shown that wind farms can have a significant negative effect on bat populations due to collisions and barotrauma. Since a few years it is also known that bats migrate regularly over the North Sea. In order to reduce uncertainties about possible impacts of the development of the offshore wind sector Rijkswaterstaat commissioned a follow-up offshore bat monitoring project as part of the WOZEP programme (Offshore Wind Ecological Programme). The monitoring will continue until at least 2019 and possibly throughout 2020. After that, the final report will be compiled, which will include a spatiotemporal analysis of the offshore occurrence of bats in all previous monitoring years. This report provides an overview of the monitoring effort and monitoring results in 2017 & 2018.Currently there are 14 monitoring locations where acoustic bat monitoring is executed, of which seven are oil & gas rigs, five are offshore high voltage stations of offshore wind farms and two are measurement platforms (Lichteiland Goeree and Europlatform). Since 2017 a new type of batdetector (Avisoft) is applied. In order to assess differences between the previously used batdetector (Batcorder) we monitored simultaneously with both detectors at three monitoring locations.The monitoring results in 2017 and 2018 show the same general pattern of occurrence as in previous years with Nathusius pipistrelle being the most common species, and Common pipistrelles and ‘Nyctaloids’ much more scarce.Furthermore the differences in performance are discussed between the currently used Avisoft detector and the previously used Batcorder. It appears that a correction should be applied in order to make measurements comparable between the two detectors, and the value of this correction factor depends on the time interval used in the analysis. At this moment the precision of the correction factor is estimated at 1.17 for 10 minute time intervals. In order to improve the precision to 1.1 simultaneous monitoring by both detectors should continue at least two monitoring years.Finally it is shown that there are obvious differences in occurrence amongst monitoring locations, between spring and autumn, as well as amongst years. However, a clear spatial pattern (e.g. from south to north or from east to west) does not emerge from this (preliminary) analysis. This should be investigated in a statistical model at the end of this project when all data of the entire monitoring period will be available as well.