The brown algae (brown seaweeds) are fascinating organisms that evolved complex multicellularity independently of animals and plants. Over the last few years, they have emerged as a highly valuable group to study the evolution of sex chromosomes and reproductive systems. I will describe how our lab has been using seaweeds to gain novel insights into the mechanisms and evolutionary trajectories of sex chromosomes and to reveal the functional and evolutionary interactions between the sex determination and key reproductive and life cycle traits.