Complexity itself is far from an evident notion. In this book, complexity of learning, learning tasks, and learning environments are discussed. With respect to learning and learning tasks, two related but different approaches to the definition of complexity can be taken. A first approach defines complexity in reference to the features of a learning task .... .... In line with specifications in the literature on learning and instruction, the different contributions in this book point to at least two aspects that should be considered more deeply in order to ‘model’ learning environments for people concerned with interests in schoolbased instruction.