This TRIO set was a 2009 Christmas gift for our 4 yr old son with some developmental delays. He loves it, and has no trouble building with this set. He needs some help with the more complicated designs, but is able to figure out most of the plans on his own after some studying. It's one of the few toys that holds his attention for more than 2 minutes. After he builds something, he then plays with, leading to some of his first imaginative play. I don't know what it is about this toy, but it is really helping our son to focus, think and plan how to build things, and to improve his overall play skills. It's amazing! And, our 3 yr old son also loves it. In fact, I'm shopping tonight to buy a bigger set, because they fight over pieces and both want to be able to build their own creation at the same time. I'm buying the 100 piece TRIO building set with storage tonight. We get more value out of this toy than I could have ever imagined. 6-yr old cousins who come to visit also settle down with this toy as soon as they arrive. I highly recommend the TRIO blocks!