Experience with the Hubble Space Telescope has shown that accurate models of optical performance are extremely
desirable to astronomers, both for assessing feasibility and planning scientific observations, and for data analyses
such as point-spread-function (PSF)-fitting photometry and astrometry, deconvolution, and PSF subtraction.
Compared to previous space observatories, the temporal variability and active control of the James Webb Space
Telescope (JWST) pose a significantly greater challenge for accurate modeling. We describe here some initial
steps toward meeting the community's need for such PSF simulations. A software package called WebbPSF
now provides the capability for simulating PSFs for JWST's instruments in all imaging modes, including direct
imaging, coronagraphy, and non-redundant aperture masking. WebbPSF is intended to provide model PSFs
suitable for planning observations and creating mock science data, via a straightforward interface accessible
to any astronomer; as such it is complementary to the sophisticated but complex-to-use modeling tools used
primarily by optical designers. WebbPSF is implemented using a new exible and extensible optical propagation
library in the Python programming language. While the initial version uses static precomputed wavefront
simulations, over time this system is evolving to include both spatial and temporal variation in PSFs, building
on existing modeling efforts within the JWST program. Our long-term goal is to provide a general-purpose PSF
modeling capability akin to Hubble's Tiny Tim software, and of sufficient accuracy to be useful to the community.