This book presents two important new findings. First, it demonstrates from first principles that turbulent heating offers an explanation for the non-adiabatic decay of proton temperature in solar wind. Until now, this was only proved with reduced or phenomenological models. Second, the book demonstrates that the two types of anisotropy of turbulent fluctuations that are observed in solar wind at 1AU originate not only from two distinct classes of conditions near the Sun but also from the imbalance in Alfvén wave populations. These anisotropies do not affect the overall turbulent heating if we take into account the relation observed in solar wind between anisotropy and Alfvén wave imbalance. In terms of the methods used to obtain these achievements, the author shows the need to find a very delicate balance between turbulent decay and expansion losses, so as to directly solve the magnetohydrodynamic equations, including the wind expansion effects.