A study by the National Highway Traffic Safety Administration found that less than 20% of children between the ages of 4 and 9 years old were properly restrained in a booster seat when riding in a vehicle.

I'm sure most of you are aware that infants need to be in the backseat in a rear-facing car seat until they are approximately 1 year old, then they can be buckled in to a forward-facing car seat (still in the back seat) until they outgrow it, usually at around 40 pounds or 4 years old. After this, they still need a booster seat until they are 4'9" tall (usually around 9 years old). All the requirements and recommendations can be found here.

And remember, the back seat is the safest place for kids up to 13 years old!