The concept of a biofuel cell, basic principles of operation, historical and present achievements as well as types of biofuel cells are described here. In principle, a biofuel cell can be considered as a special type of an electrochemical fuel cell in which, instead of noble-metal (e.g. Pt) type catalytic electrodes, biocatalysts in a form of microorganisms or enzymes are used. Consequently, biofuel cells operate under milder conditions (neutral electrolytes, ambient temperature and pressure) in comparison to conventional fuel cells. Typically, such biofuels as ethanol, glucose, formic acid or lactic acid are utilized on the anodic side. To make biofuel cells practically more attractive, there is a need to improve cathode and develop stable and effective bioelectrocatalytic systems for the oxygen reduction. Depending on whether the biocatalyst (enzyme) exists in living organisms (bacteria) or in the isolated form, biofuel cells can be divided into microbial and enzymatic ones. Another important issue is whether the biocatalyst is immobilized on the electrode surface or it is dissolved in solution (and subject to diffusional mass transport). Further, depending on a mechanism of charge propagation and distribution (electron) to the biocatalytic active sites, biofuel cells can be distinguished in terms of systems utilizing mediators (MET or mediated electron transfer type) or operating without mediators (DET or direct electron transfer type). Recently, there has been growing interest in biofuel cells because of the environmental concerns and, primarily, due to the necessity of searching for novel alternate energy resources.