Most answers above are correct per HW spec, but BIOS cannot support all possible combinations. FW may run out of code space, tests may miss some corner cases, or it may be deadline to ship. Here are some simple tips to save you the trouble to return incompatible RAM. :) (1) If your Mac/PC requires DDR3L 1.35V, please use DDR3L. Do not use DDR3 (1.50V only). (2) If your Mac/PC requires DDR3 1.5V, you may use DDR3 or DDR3L (dual 1.35V or 1.50V). (3) If you mix DDR3 and DDR3L, put DDR3 in the first slot (find lowest alphabet or number). (4) If your Mac/PC requires PC3-8500 (1066MHz), it can probably use PC3-10600 (1333MHz). (5) If your Mac/PC requires PC3-10600 (1333MHz), it can probably use PC3L-12800 (1600MHz). (6) If you mix different speeds (MHz/RAS/CAS), please put the slower RAM in the first slot.