Co-simulation of human-exoskeleton systems can be a very powerful way to assess the effects of an assistive device. However, it can be challenging to develop models for co-simulation. This introductory webcast on using musculoskeletal models to design assistive device will talk about co-simulation of human-exoskeleton system in two parts. In the first part, the webcast will show how musculoskeletal models can be used in different stages of the design process. In the second part, we will talk about the different methods to kinematically and kinetically connect the human and exoskeleton models.