Why is 月 used when speaking about anatomy? One thing I've been curious about while learning Chinese is the use of 月 in many 汉字 representing anatomical features (for example, 脸， 腰， or 腿).

What is the etymology of the use of 月 as a radical?
The 月字旁 was originally '肉' &amp; not '月' - 肉 has the meaning of 肉体 meaning 'flesh' or having to do with the 'human body' so it's often seen with body parts.
Wikipedia:
肉字旁：臺灣標準中，凡肉字旁的字，都寫作「提肉旁」即，使其不會與「月字旁」相混。《字形表》中，肉字旁只在字的左旁時才寫作「提肉旁」，在字的右旁時採用首筆豎的方式與「月字旁」區分（「月字旁」在右方時，首筆為撇）。但在下方時，則「肉」與「月」首筆都作豎，兩者會相混。