  At the dawn of thermodynamics, Carnot's constraint on efficiency of heat
engines stimulated the formulation of one of the most universal physical
principles, the second law of thermodynamics. In recent years, the field of
heat engines acquired a new twist due to enormous efforts to develop and
describe microscopic machines based on systems as small as single atoms. At
microscales, fluctuations are an inherent part of dynamics and thermodynamic
variables such as work and heat fluctuate. Novel probabilistic formulations of
the second law imply general symmetries and limitations for the fluctuating
output power and efficiency of the small heat engines. Will their complete
understanding ignite a similar revolution as the discovery of the second law?
Here, we review the known general results concerning fluctuations in the
performance of small heat engines. To make the discussion more transparent, we
illustrate the main abstract findings on exactly solvable models and provide a
thorough theoretical introduction for newcomers to the field.
