Is it better to put plastic on the inside or outside of windows?

Whether it is better to put plastic on the inside or outside of windows for insulation depends on climate and purpose, but generally inside is better for most homeowners. Putting plastic film on inside of windows using shrink film kits with double sided tape and hair dryer to create airtight insulating air gap is most effective for winter draft reduction, easier to install, protected from wind and rain, lasts longer, and creates better seal because interior air is less humid and tape adheres better to warm interior trim. Interior application also allows you to still open exterior storm windows and does not get damaged by weather. Outside plastic can be effective if interior trim is too damaged to hold tape, or for very leaky windows where you want to stop wind before it enters, but exterior plastic is exposed to wind, rain, UV, and can tear quickly, looks worse from outside, and may trap moisture between plastic and window causing mold. For security or storm protection, exterior plywood or security film on inside of glass is used. Best practice is interior shrink film kits for winter insulation, ensure window is clean, apply tape to trim not glass, shrink tight, and remove in spring to avoid moisture trapping long term.