We study the exchange interactions in half-metallic Heusler alloys using first-principles calculations in conjunction with the frozen-magnon approximation. The Curie temperature is estimated within both mean-field (MF) and random-phase-approximation (RPA) approaches. For the half-Heusler alloys NiMnSb and CoMnSb, the dominant interaction is between the nearest Mn atoms. In this case, the MF and RPA estimations differ strongly. The RPA approach provides better agreement with the experiment. The exchange interactions are more complex in the case of full-Heusler alloys Co₂MnSi and Co₂CrAl where the dominant effects are the intersublattice interactions between the Mn(Cr) and Co atoms and between Co atoms at different sublattices. For these compounds we find that both MF and RPA give very close values of the Curie temperature slightly underestimating experimental quantities. We study the influence of the lattice compression on the magnetic properties. The temperature dependence of the magnetization is calculated using the RPA method within both quantum mechanical and classical approaches.