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loop is Common Lisp's iteration powerhouse — practically a mini-language.

Counting:

(loop for i from 1 to 10
      do (print i))

(loop for i from 10 downto 1 by 2
      do (print i))

Iterating over collections:

(loop for x in '(a b c d)
      do (print x))

(loop for x across #(1 2 3)             ; vector
      do (print x))

(loop for x being the hash-keys of ht
      do (print x))

Collecting results:

(loop for n from 1 to 5
      collect (* n n))                    ; (1 4 9 16 25)

(loop for n in '(1 2 3 4 5)
      when (evenp n)
      collect (* n n))                    ; (4 16)

Accumulators:

• collect — build a list
• sum / count / maximize / minimize
• nconc — destructive list concat

(loop for n in '(1 2 3 4 5) sum n)        ; 15
(loop for n in '(1 2 3 4 5)
      when (evenp n) count n)              ; 2

Multiple iterators:

(loop for x in '(a b c)
      for y in '(1 2 3)
      collect (list x y))                  ; ((a 1) (b 2) (c 3))

Conditions:

(loop for n from 1 to 100
      while (< n 50)
      sum n)

(loop for n from 1
      until (> n 100)
      do (print n))

loop reads almost like English. There's also a do macro (more parens, less English) and dolist/dotimes for simple cases.