Digital health information is widely available, but not everyone fully benefits due to limited health literacy. Until now, little was known about how health literacy influences information processing and how design features of digital health information can be used to create optimal health messages for different health literacy groups. This dissertation showed that processing health information places more cognitive demands on people with limited health literacy, resulting in less recall of the information and less positive attitudes towards the message compared to people with de adequate health literacy. Also, people with limited health literacy find it more difficult to imagine the content of health information, resulting in less positive attitudes. Optimally designed messages are therefore low in cognitive load and appeal to the imagination. Such messages are composed by non-difficult texts, illustrated, offer spoken text, and use animations. Furthermore, information that is suitable for people with limited health literacy appeared to be effective in adequate health literacy groups as well. Health communication professionals should therefore not be hesitant to apply these design features in their materials. Carefully designed health messages improve information processing in limited health literacy groups, resulting in a more knowledgeable population and better informed health decisions.