For parents, it's a process that's not only exhausting but nerve-wracking as well.
What should be happening at what age and when is it time to be concerned?
"There can be a range of several months in which the development of a new skill is entirely normal," Dr. Eileen Costello of Brigham and Women's Hospital said.
One of the first and most anticipated milestones is a baby's first true or "social" smile. It usually happens around two months.
"A social smile is when you work at it a little bit, and the baby, suddenly, its whole face lights up," Costello said.
At two to three months, babies should be able to lift and support their own heads.
Soon after come their first attempts at talking.
"A four-month-old will be making AH, EE, OO - those kinds of sounds," she said.
Actual words will come later at around 12 to 15 months.
Most babies roll over at six or seven-months-old, crawl at nine-months-old and walk at about 12 months.
No two babies progress at the same rate but there are certain red flags that may indicate developmental problems.
Parents should consult a doctor if at three months the baby lacks head control and appears floppy in parents' arms, or if the infant doesn't smile, respond to noise or has trouble with eye contact.