The Head of State, or the Head of Government in the case of countries like Australia where they are two different people, is immune from prosecution for criminal acts.

After heads of state are no longer in office they can be prosecuted for crimes allegedly committed before or after their time in office, but they cannot be charged with crimes committed as acts of state.  Some senior ministers are covered by a similar immunity.

Recently, more exceptions have been made to this, and the immunity does not apply to International crimes like Crimes against Humanity, Genocide and War Crimes.  The logic behind these exceptions is that these crimes cannot be acts of state.

In some situations, a head of state cannot claim immunity against a charge of torture.

However, I would question the whole Head of State Immunity idea.  In my opinion, Crimes should NEVER be  committed as acts of state.  If my opinion is accepted, then head of state immunity is meaningless.

Of course, heads of state would need protection against malicious or time wasting prosecutions, but so does everyone else.