In this talk, I will argue that the rapid growth in the last decade of direct public participation in science via the Web – what I call citizen cyberscience – has important implications for the open science agenda. Leading researchers now routinely use citizen cyberscience to tackle large-scale scientific computing and data analysis challenges, in areas as diverse as climate science, epidemiology and molecular biology. In aggregate, millions of volunteers are contributing to such projects by donating spare time on their computers, participating directly in data analysis via the Web, or even collecting data from the field using smart phones. In exchange for their effort, volunteers typically want more openness and better communication about what the scientists are doing. Some even want to have an influence on the direction of the research. This raises new issues for how open and participative the scientific process can and should be.