Short introduction and history.

If one does a MEDLINE search using as keywords chemo fog or chemo brain or their hyphenated equivalents, fewer than 30 'hits' appear. The oldest dates back to 2003. This small number of hits in some way captures one aspect of the current state of the phenomenon (or phenomena). In contrast, if one does the search using 'cognitive x cancer x chemotherapy', hundreds more hits appear. This in some way captures another aspect of the phenomenon.It is both little-known and well-known. To go a step further, some data suggest that it is one of the most common adverse effects of chemotherapy, other data suggest that it does not exist. Even its name (or lack thereof) is still unsettled. Yet, patients consistently report it. This chapter introduces the reader to the fascinating and complex challenges-to patients, healthcare providers, basic scientists, employers, insurers and others-inherent in this topic and the current state of knowledge about it.