The current research interest in the problem of underemployment is associated with recent changes on the labor market, in particular, the growth in the so-called alternative or flexible employment relations. Today, determining whether and under what conditions a growth in the proportion of workers employed on the basis of atypical contracts results in a deterioration of work conditions poses an important (also from a practical point of view) question for labor market studies. Since the current controversion surrounding this issue can be attributed to the vagueness of the term underemployment, the first part of this article presents and analyses different ways in which this term is defined in the labor market literature. The second part analyses the relationships between underemployment and flexible employment. In the third part, The authotress presents the results of statistical analyses of data from Polish survey research. The object of these analyses was to determine the extent to which young people (under 30 years of age) in Poland are affected by underemployment.