If you overheat at idle, but not at highway speed, it is often an indication
that your
fans are not working.

Conversely, if you overheat at highway speed but not at idle, it tells you
that the
increased heat generated by the engine is not being adquately removed.  And,
some
of the reasons are:
(1) slipping water pump belt, or bad water pump
(2) radiator hose on the suction side of the water pump collapsing
(3) partially blocked radiator
    (3a) check for bugs, grass clippings, mud in the radiator, particularly
between the
     A/C condenser coil and the radiator face.. A blocked condensor coil
alone will
     keep air flow from the radiator.
    (3b) radiator partially blocked internally
(4) system not completely filled with coolant.  Check radiator expansion
cap, fill system
    properly to exclude air pockets, etc.  If the system 'uses' coolant,
check for leaks
    near hoses, at the radiator, from 'freeze' plugs, head gaskets, etc.

A blown head gasket, or cracked head or block will also cause overheating
and can,
in worst cases, be a bit tough to localize.

I may have missed a few, but this should get you started.