The size 3m fit my 6 month old son. It still fit at 10 months, but we entered colder weather and had to put the shoes away. What is so great about Toms is the one for one (a pair of shoes is donated for every one you buy). I love the design for the baby Tom because the velcro and snug fit makes it easy to put on and hard for a baby to get off. This was the first shoe that was always on my son's foot until we took it off. Buying advice: Go a size larger so your baby has room to grow in them (unlike traditional Toms) because the velcro is very strong and the shoe has a snug fit.