Traditionally, knowledge is modelled in epistemic logic as lack of uncertainty. On the other hand, lack of knowledge is often called ignorance. But, ignorance and uncertainty are different concepts. So, how should we model ignorance and formalise reasoning about ignorance? How ignorant can an agent be? How mutually ignorant can two agents be? I will discuss these and related questions, as well as some challenges in capturing and formalizing adequately the relationship between knowledge and ignorance.