Look for the following information on diode data sheets ...

• Maximum forward current - your circuit must pass less current than this allowing a safety margin.
   
• The voltage drop across the diode when it is conducting. In some circuits this can be important.
   
• Maximum reverse voltage - your circuit must not exceed this value or the diode might break down. Also remember that with AC circuits the peak voltage must be used.
   
• The switch-on current. This can be much larger than the average current becuase large capacitors might have to charge up.
   
• With high frequency circuits, the diode's capacitance might become important. If the diode is behaving like a capacitor, it will not block currents in one direction so effectively.