KSP's most established colony/base mod, MKS provides a huge array of new gameplay systems and supporting parts to support the experience of designing, building, and maintaining interplanetary colonies in Kerbal Space Progam.

Interplanetary Colonization represents one of the largest technological and logistical challenges on the horizon for manned spaceflight.

For over three years, MKS has been the definitive Colonization mod for KSP, with continued refinement of the parts and core systems (including the resource system now integrated in the stock game) based on extensive community feedback and continued development and design.

MKS is not a parts pack. It is a gameplay mod that adds a massive amount of end-game content for KSP, and can serve as the perfect capstone for your career save. It can be soul crushingly hard, and as unforgiving as gravity. But it can also be an incredibly rewarding gameplay experience, bringing back that sense of wonder and accomplishment that many of us remember from our first Mun landing.

If you want a parts pack where your idea of a colony is landing a single ship and call it a day, this is probably not the mod for you. If you want something that transforms your gameplay, and gives you unapologetically difficult goals where you have to juggle planning, logistics, and the harsh constraints imposed by space, then you will likely get as much enjoyment playing MKS as I have received building and designing it.

Features

MKS adds a rich array of new features to support the challenge of building interplanetary colonies, including:

A huge assortment of parts suitable for surface and orbital bases and stations, drawing inspiration from real-world parts and concepts from Bigelow, NASA, and others.
A clear progression of parts, from simple outposts and mobile bases to larger manufacturing complexes and permanent colonies.
Extended ISRU operations with more resources and more complex harvesting, refining, manufacturing chains, and new mechanics.
Discoverable resource lodes for more interactive resource harvesting and exploitation
Customizable and field-configurable converters and harvesters to allow post-launch customization of your colony.
Tools for handling large bases, such as ground tethering and automated resource transfers for disconnected bases
Tools for building out your base In-Situ with harvested resources, whether it's by expanding and kitting out modules, or through in-situ construction via the bundled Ground Construction mod by @allista
Parts and tools for contructing bases including weldable ports, magnetic couplers, and heavy construction equipment via the bundled Konstruction mod.
Parts for mobile bases via the bundled Karibou Expedition Rover
Expanded Kerbal traits and effects in support of Kolonization
Comprehensive and seamless integration with USI Life Support, including life support, habitation, and homesickness.
Kolony rewards, providing ever increasing efficiencies as well as tangible bonuses in career mode, as your Kerbals gain experience colonizing other planets and moons.
The ability to support permanent colonies as well as ark-ships as your Kerbals expand their knowledge and become more experienced with off-world colonies.
Part disassembly and recycling
Wireless power transmission and Geothermal wells
Equipment wear and tear along with manual and automated maintenance tools
And much more!

FAQ

I hear MKS is really complicated - is that true?
MKS has significant depth and breadth, but provided you don't try to do everything at once, you will find the learning curve can be pretty reasonable. Start with life support, and don't get fixated on trying to build everything at once. The patterns learned with a small base quickly scale for larger ones.

What about documentation?
Thanks to @dboi88, MKS enjoys full KSPedia documentation.
There's also an excellent community maintained wiki available here.
MKS Documentation even has its own development thread here.

I can't seem to fit a 100% self sufficient base in a single launch - what am I doing wrong?
Building a self sufficient colony is hard. Like... REALLY hard. It's going to take a lot of launches and a lot of work. The idea of MKS is that it is not about plunking down a base and being done. Rather, it's the journey to get to that point that is the fun bit. So yeah... you are going to need a few more rockets.

I can't find a place to create my base that has all of the resources!
Good. Now you get to enjoy some of the challenges that result from resource constraints. Do you ship things in? Do you set up multiple bases? Or use low-efficiency harvesting to get trace materials and salvage the rest from scrapped components?

I feel overwhelmed... I try to make a base, and it seems I just need so many different parts!
Start small. Start with just a habitation module or two and ship in supplies. Then add some agroponics and ship in fertilizer. Once you get those down pat, you are ready for more complex systems.

I am still lost - where can I get help?
Ask in the official forum thread (linked below). A lot of the folks have been using MKS in their games for years, and are a pretty friendly lot.

*NOTE: FOR SUPPORT ISSUES, CLICK HERE TO VISIT THE OFFICIAL KSP FORUM THREAD!*

USI Kolonization Systems (MKS/OKS)