What is the most important structural difference between an HMO and a PPO health insurance plan?

The defining feature of an HMO (Health Maintenance Organization) is the requirement to select a primary care physician who coordinates all your healthcare and must provide referrals before you can see specialists. HMOs generally have lower premiums and no out-of-network coverage except in emergencies. A PPO (Preferred Provider Organization) allows members to see any provider in or out of network without referrals, at the cost of higher monthly premiums. EPOs and POS plans fall between these two models in terms of flexibility and cost.