My daughter is now 18 months and she still isn't walking. Should I be concerned?

Yes. I strongly encourage you to seek medical care for your daughter. Not walking by 18 months is definitely gross motor delay. She needs to be evaluated.
Yes. After 15 mo the kid not walking independently is outside the acceptable range for motor development.Some delay may be seen in premi's, but i would first do a complete exam looking for appropriate reflex development & then send the child to the local developmental center for pt & ot evaluation.Developmental therapy focused on suppressing primitive reflexes & encouraging muscle use can help.
Speak to your doctor. 18 months is considered the extreme of average for walking. Speak to your pediatrician about your concerns. Is she pulling to a stand and cruising holding on, and her other developmental milestones such as language and socialization are in the average range, then expect walking to come shortly.