The sequential aspects of molecular logic are taken up in this Chapter, even though the need to avoid memory effects in some of the combinational logic devices was mentioned in several previous Chapters. Some of the simplest molecular memories can be seen to emulate the R-S Latch. The somewhat more complex D Latch also has molecular versions. Molecular keypad locks are perhaps the most complex devices of this kind available so far. The order in which the inputs are presented to the device takes on special importance here. While several approaches are available to achieve molecular keypad locks, many multi-step chemical syntheses are shown to have the same logical properties.