Lenalidomide (Revlimid®, also known as CC-5013) and pomalidomide (CC-4047) are IMiDs and chemical derivatives of thalidomide. Lenalidomide was introduced in 2004 and is currently approved for treatment of multiple myeloma and 5q-myelodysplastic syndromes (MDS). In addition, IMiDs are currently tested in a wide variety of haematological as well as solid tumours. IMiDs have three main anti-tumour properties, i.e. a direct anti-tumour effect, an inhibitory/modulatory effect on the tumour stroma/microenvironment supporting growth and survival of tumour cells including anti-angiogenic properties as well as a very potent immunomodulatory effect. The latter includes activation of T, NK and NKT cells as well as modulation of the function of mononuclear cells (DC and monocytes). However, the exact anti-cancer mechanisms of IMiDs in vivo remain elusive so far. This brief review will focus on the current concepts explaining the anti-cancer effects of IMiDs.