Determine the number of significant digits in the value 100.1 .

The given value is $100.1$.

According to the rules for determining significant figures, all non-zero digits are significant, and any zeros between non-zero digits are also significant.

In the number $100.1$, the digits $1$ are non-zero, and the two zeros are trapped between them.

Thus, all four digits are significant.

The number of significant digits is $4$.