Fluctuations in cognition and alertness (FC/FA) are key manifestations of dementia with Lewy bodies (DLB) and also have been recognized recently in patients with Parkinson's disease (PD) with dementia, a condition that shares important clinical, genetic, and neuropathologic characteristics with DLB. A comprehensive assessment of potential episodes of FC/FA is required for adequate clinical management, and several interesting clinical instruments are being developed for that purpose. FC/FA should be differentiated from episodes of excessive daytime sleepiness (EDS). Such diagnostic differentiation appears to be necessary, particularly in the light of the different therapeutic approaches to FA and EDS. Based on the deficit in cholinergic transmission observed in DLB patients, cholinesterase inhibitors, such as rivastigmine, may have a beneficial effect on FC/FA. Other therapies, such as melatonin or modafinil, require further investigation.