Currently we have five main sets in our program, Ki 0, Ki 1, Ki 2, Ki 3 and Ki 4. Then there is also the Ki 3a set which upgrades the Ki 3 set to a Ki 4 set. The main difference between the sets is that the larger kits provide you with more parts, enabling your child to build larger and more complex models. The Ki 3 and Ki 4 sets also contain rounded and sloped parts. The Ki 4 set (and the Ki 3a) have parts which can be used as a bearing for a crank shaft.