Engineer/TL is the IQ/H skill of designing and building technology, on all scales. For GURPS, with its underlying bias towards a rational-scientific worldview, this is a big deal: this is how people turn the laws of the universe into the things they want to make and do. Having a single skill for this for all physical, non-living things (Bioengineering and Computer Programming do the same for biotechnology and software respectively) makes a statement about the presumed ability of rational creatures to deal with the universe. Engineer requires specialisation, but most of the specialisations default to each other at -4: characters can be an Edison or a Brunel, working across several fields and changing the world through construction or inventions. Versatile and Hidebound respectively help and hinder inventing. Engineer takes equipment modifiers, and at high TLs, often requires software tools to work at full effect.
Engineer always has a prerequisite of Mathematics (Applied) at TL5+, and specialisations may have prerequisites of their own, usually Physics, but sometimes Chemistry or Metallurgy. Specialisations have individual defaults to Repair or Use skills, some of which are a little strange for historical
reasons. Architecture, Armoury, Electrician, Electronics Operation, Electronics Repair, Explosives (Demolition), Mechanic, and Mathematics (Applied) have defaults to appropriate Engineer specialisations.
Unlike Electronics Operation or Repair, the list of specialisations is not prescriptive, and you're expected to extend it for campaigns with odd technologies. One unlisted specialisation that has existed for several TLs and is still going strong is (Chemical), which would have a prerequisite of Chemistry and a default to Professional Skill (Chemical Plant Operator).
The commonest specialisation, by far, on GURPS templates is Engineer (Combat), with (Small Arms) and (Mining) following;
LTC3 describes the work of mining and
Underground Adventures has more for higher TLs.
DF has Engineer (Gadgets), which seems to cover a rather wide range of technology.
High-Tech and
Ultra-Tech have things characters can do with several Engineer specialisations, and
Low-Tech touches on the rise of engineering.
Horror has Engineer (Spirit), and
Monster Hunters illuminates the importance of Engineering skill if you want to be a true expert in a technology. As usual for skills,
PU3 and
PU7 have examples of applicable Talents and Wildcard skills.
Powers and
Enhanced Senses cover the uses of Engineer to interpret information from unusual senses, or use powers
well.
Thaumatology,
Chinese Elemental Powers and
Urban Magics consider Engineering applied to magic.
One of the major uses of Engineering is the invention rules. These work pretty well for present-day technology, provided you remember something important. Most machines consist of lots of parts that were invented separately. Nobody in a realistic setting invents a completely new car, or gun. They invent a new part, improve a few parts at a time, or combine existing parts to a new purpose. As long as you remember that fiction
never portrays realistic invention, the rules seem pretty solid.
What have you done with Engineer in a game that was awesome? Am I the only GM who's run an edisonade in GURPS?