Mathematica's Null is interpreted as R NULL during conversion to RLink's internal representation:

Of course, it also works in another direction:

You can set a variable (or part of an expression, e.g. part of a list) to NULL:

Comparison to NULL does not really work—it returns a logical vector of length 0:

You can, however, test the type of the value stored in the test variable:

The proper way to test for NULL is via the is.null built-in R function:

Vectors of zero length are also considered NULL in R:

For this reason, assigning NULL to a vector's element is an error:

Assigning NULL to an element of a list is not an error, and effectively deletes that element from a list. Here is a test list:

This will assign NULL to its second element:

This is equivalent to removal of that element from a list:

In some sense, R NULL therefore combines the behavior of Mathematica's Null and Mathematica's Sequence[].