To find the sum of the first 100 natural numbers, you can use the formula for the sum of an arithmetic series:

\[ S_n = \frac{n}{2} \times (a + l) \]

where:
- \( S_n \) is the sum of the series,
- \( n \) is the number of terms,
- \( a \) is the first term,
- \( l \) is the last term.

For the series 1 + 2 + 3 + ... + 100:
- \( n = 100 \),
- \( a = 1 \),
- \( l = 100 \).

Plugging these values into the formula gives:

\[ S_{100} = \frac{100}{2} \times (1 + 100) \]
\[ S_{100} = 50 \times 101 \]
\[ S_{100} = 5050 \]

So, the sum of the first 100 natural numbers is 5050.