Assert
Concurrent Statement ---- used in ----> Entity
Architecture
Sequential Statement ---- used in ----> Process
Function
Procedure

Syntax

assert condition report string severity severity_level;

See LRM sections 8.2 and 9.4

Rules and Examples

The assert statement tests the boolean condition. If this is false, it outputs a message containing the reopirt string to the simulator screen: 
assert (J /= C) report "J = C" severity note;

The severity level may be defined as note, warning, error or failure Level failure normally aborts the simulation. 
assert not OVERFLOW) report "Accumulator overflowed" severity failure;

If the message clause is ommited, a default message is output. The severity level and the name of the design unit containing the relevant assert statement may also be output

If the severity clause is ommited, the default level is error.

A concurrent statement monitors the boolean condition continuously

An unconditional message can be output by using the literal false: 
procedure PUT
	(signal STACK  : inout T_STACK;
	signal POINTER : inout T_POINT;
	signal ITEM    : in    T_DATA) is
begin
	if (POINTER < 5) then
		STACK(POINTER) <= ITEM;
		POINTER <= POINTER + 1;
	else
		assert false report "Stack overflow" severity error;
	end if;
end PUT;

As well as functional errors, timing errors can be reported via assert: 
CHECK_SETUP: process (CLK, D)
begin
	if (CLK'event and CLK = '1') then
		assert D'stable(SETUP_TIME) report "Setup Violation..." severity warning;
	end if;
end process CHECK_SETUP;

Synthesis Issues

Assert statements are ignored by logic synthesis tools.

Whats New in '93

In VHDL-93, the assert statement may have an option label.

A concurrent assert statement may be run as a postponed process

VHDL-93 allows report to be used on it's own as a sequential statement, giving the same functionality as assert false, except that the default severity is note

MSG1: report "Starting test sequence" severity note;