Part customisation is the process whereby stock elements are altered by a user to better suit a specific purpose. These can be prototpypes or inspirations for new custom elements being developed which fit within the LEGO system or simply one-off creations for use in a single MOC
Sometimes a builder may wish to perform a task which standard elements from their chosen building system cannot manage. For example, they may wish to light up a street lamp or use a baseplate in an unavailable dimension. Most AFOLs will simply satisfy themselves with existing solutions or workarounds, but some builders however will alter some elements in order to meet their specific requirements.
Part alteration ranges from small changes such as cutting a baseplate through to major operations, such as changing more than one LEGO train track into a new, more versatile form.
Sometimes, part alterations are carried out by groups using bricks for rapid-prototyping of robotics applications. Examples of this include creating spring-loaded electrical contact systems, sensor mounts, and bricks with embedded magnets.
While the typical minifig comes in a wide variety of forms, sometimes a user may wish to reproduce a favoured comic book character, create a historical figure, or otherwise assemble a custom minifig which goes beyond the usual range or parts. One solution to this is to paint, cut, apply stickers or decals and otherwise alter an existing minifig until it meets ones requirements. Brasso is often used to remove printing.
This is perhaps the most common part customisation and some AFOLs make a regular habit of it, sometimes even selling minifig representations of popular characters. For some good examples see  and . The Minifig Customization Network (or MCN) exists as an online community for customizers.
Customised parts are generally eschewed by purists.