When fermions interact with nontrivial topological backgrounds, like Solitons, or are constrained by boundary conditions, the second quantized fermion number operator of the system can have fractional eigenvalues.At zero temperature, the induced fermion number is a topological quantity, i.e, is independent of the detailed profile of the background field and depends only on its global, asymptotic values.At nonzero temperature, I analyze field theory models of increasing complexity in (1+1), (2+1) and (3+1) dimensions and show that the induced fermion number is generically nontopological and is not a sharp observable, i.e, has nonvanishing rms fluctuations.