  We present an updated cumulative size distribution (CSD) for Jupiter Family
comet (JFC) nuclei, including a rigourous assessment of the uncertainty on the
slope of the CSD. The CSD is expressed as a power law, N(>r_N) \propto
r_N^{-q}, where r_N is the radius of the nuclei and q is the slope. We include
a large number of optical observations published by ourselves and others since
the comprehensive review in the "Comets II" book (Lamy et al. 2004), and make
use of an improved fitting method. We assess the uncertainty on the CSD due to
all of the unknowns and uncertainties involved (photometric uncertainty,
assumed phase function, albedo and shape of the nucleus) by means of Monte
Carlo simulations. In order to do this we also briefly review the current
measurements of these parameters for JFCs. Our final CSD has a slope q=1.92\pm
0.20 for nuclei with radius r_N \ge 1.25 km.
