Insufficient levels of physical activity increase the risk of developing cardiovascular disease, some cancers, diabetes, osteoporosis, depression, anxiety and obesity. However, less than half of the Australian population meet the minimum physical activity guidelines of accumulating 30 minutes of moderate intensity physical activity on most days of the week. This increases the burden of disease, lowers quality of life and costs the health care system over AUD 719millionperyear.Therefore,thereisanurgentneedforeffectivepopulationbasedinterventionstoincreasephysicalactivityatlowcostforlargenumbersofpeople.TheInternetprovidesagoodplatformtodeliverphysicalactivityinterventionsasitcanreachlargenumbersofpeopleatlowcost.Whilsttheshortβtermeffectivenessofwebβbasedinterventionshasbeenestablished,effectivepromotionofwebβbasedinterventions,aswellaslongβtermparticipantengagementandretentionintowebβbasedinterventions,canbeproblematicandneedstobeaddressedinordertoimprovethelongβtermeffectivenessofthesekindsofinterventions.Therefore,thefirststudyexaminedthecostβeffectivenessofwebβbasedadvertisingmethodscomparedtotraditionalprintβbasedadvertisingmethods,astheyhavethepotentialtoattractlargenumbersofpeopleintoawebβbasedphysicalactivityinterventionatalowercost.Forthesecondstudy,a3βgrouprandomisedcontrolledtrialwasconductedtodeterminetheeffectiveness(intermsofretention,adherence,websiteengagement,satisfaction,physicalactivitychangesandqualityoflifechanges)ofusingonlinevideoβcoaching(usingSkype)inadditiontopersonallyβtailoredphysicalactivityadvice.Forthethirdstudytheeffectivenessofvideoβtailoredadvicetoimproveattentionandrecallofthephysicalactivitymessagewascomparedtobasictextβtailoredadviceusingeyetrackingtechnologyandarecallquestionnaire.Findingsfromtherecruitmentevaluation(study1)revealedthatthecostβeffectivenessofboththewebβbasedandprintβbasedmethodsvariedsubstantially.Newspaperarticlesandcommunitycalendarscosttheleastpersignβup,butresultedinasmallnumberofsignβups(17and6respectively).ThetargetedFacebookadvertisementswerethenextmostcostβeffectivemethod(AUD45 per sign up), and reached the most number of sign-ups (184). People reached through the targeted Facebook advertisements were on average older and had a higher BMI than people reached through the other methods. Google advertisements and newspaper advertisements were not cost effective. Further research is needed to determine the effectiveness of Facebook advertising for attracting specific population groups and evaluate the use of mass-media to attract larger numbers to population level interventions. The intervention trial (study 2) revealed that the tailored advice + video-coaching group significantly improved their physical activity in comparison to a wait-list control group. However due to a low adherence to the coaching sessions, the tailored advice + video-coaching group did not improve their physical activity more than the tailoring only group. Participants who participated in the video-coaching sessions were nonetheless satisfied and had higher program and website engagement. Further research using online video-coaching should investigate ways to improve coaching adherence. The eye-tracking study (study 3) demonstrated that video-tailored advice leads to improved user-engagement compared to text-tailored advice (i.e., video participants paid more attention and interacted with the website for longer). However no group differences in recall of the physical activity message were found. More research is needed to determine how recall of computer-tailored advice can be improved and whether video-tailored advice can lead to greater health behaviour change than text-tailored advice. In summary, the findings from this PhD add valuable knowledge to the literature about improving the promotion, engagement and effectiveness of web-based physical activity interventions, and inform the development of the next generation of interventions