  In this review we address the uncertainties implicit in evolutionary
synthesis model computations. After describing the general structure of
synthesis codes, we discuss several source of uncertainties that may affect
their results. In particular, we discuss the uncertainties arising in the
computation of isochrones from evolutionary tracks; those related to atmosphere
models; those that are a consequence of the incompleteness of the input
ingredients; and those associated with the computational aspect used in
synthesis codes. We also discuss the issue of the inclusion of distributed
properties in synthesis models; as a paradigm of this case, we illustrate the
difficulties implied by the inclusion of tracks with rotation in synthesis
models. Finally, we describe several examples of the statistical approach to
population synthesis. We report on the failure of the fuel consumption theorem
(FCT) and the isochrone synthesis code to produce mutually consistent results.
However, we argue that FCT and isochrone synthesis results are reliable for
application to real systems in the wavelength range where they coincide. On the
constructive side, we derive several useful survival strategies to bypass
uncertainties. We show that single stellar populations at the turn-off ages of
the tabulated tracks can be safely compared, as they are scarcely affected by
the interpolation scheme used to compute isochrones. Finally, we suggest to use
derivative quantities, such as the SN-rate, as bug detectors. On the
recommendation side, we advocate for greater transparency and more
documentation in synthesis modeling. We also ask stellar model makers to think
of us and include more mass values in the tracks.
