From my experience and what I've read, it's pretty normal for kids to go through some separation anxiety in conjuction with different developmental milestones. 18 mths, plus or minus a couple of months, is one of those times. The feelings are exacerbated by the fact that their language skills are still limited at this point depsite that their ability to feel is fully engaged. My husband travels a lot and it was at about this age when our daughter started having some difficulty. A teacher recommended that we put a picture of our family (including our dog) in a a zip lock bag and give it to her to carry around (the bag is to protect the picture from all the inevitable drool, etc.). Whenever she felt sad, they'd get out the baggy, give it to her, and start talking about her daddy, what she did with him, that he was coming back and when, etc. This allowed her to "express" her feelings and also feel like he was with her b/c she had a physical object (the picture of him) to hold in her hands. She even slept with it at naptime once in a while when she was feeling particularly sad. We did this with much success at her daycare. I don't see why you couldn't do the same at home.