Find the missing number.
13, 1, 17, ?, 21

The given series is an alternating series consisting of two separate sub-series:
1. First sub-series (at odd positions): 13, 17, 21
* Pattern: $+4$ ($13+4=17$, $17+4=21$)

2. Second sub-series (at even positions): 1, ?
* Following the same pattern of adding 4: $1+4=5$

Therefore, the missing number is 5.