This book covers important practical approaches including efficient Bayesian updating with context tree weighting, and stochastic planning, approximated by sampling with Monte Carlo tree search. Algorithms are also included for the reader to implement, along with experimental results to compare against. This serves to approximate AIXI, as well as being used in state-of-the-art approaches in AI today. The book ends with a philosophical discussion of AGI covering the following key questions: Should intelligent agents be constructed at all, is it inevitable that they will be constructed, and is it dangerous to do so?