We report the density-functional-theory calculations of the exchange interactions and Curie temperature for a series of III-V, II-Vi diluted magnetic semiconductors (DMS) and Co-based half-metallic full Heusler alloys. The calculation of the exchange parameters is based on the frozen-magnon approach. The Curie temperature is calculated within the mean-field approximation to the classical Heisenberg Hamiltonian. For DMS we focus the discussion on the role of holes in establishing the ferromagnetic order in various systems. Our calculations of the exchange interactions for Heusler alloys show that in this sstem the inter-sublattice exchange interactions play the main role in the thermodynamics of the system.