To evaluate the series \(1 - 2 + 3 - 4 + \ldots + 99 - 100\), we can group the terms in pairs:

\[
(1 - 2) + (3 - 4) + \ldots + (99 - 100)
\]

Each pair \((2n-1) - 2n\) simplifies to:

\[
(2n-1) - 2n = -1
\]

There are 50 such pairs in the series because the sequence goes from 1 to 100, which contains 100 terms, and each pair consists of 2 terms.

Therefore, the sum of the series is:

\[
50 \times (-1) = -50
\]

So, the value of the series \(1 - 2 + 3 - 4 + \ldots + 99 - 100\) is \(-50\).