In this longitudinal study, we compare the age of reaching early developmental milestones in bilingual and monolingual children and between the bilinguals' two languages. We present data from 302 Polish bilinguals (living outside of Poland with various majority languages) and 302 Polish monolinguals, aged = 12.78 months on study entry (range: 0 24 months), matched on sex, age at study entry, duration of parental reporting, and parental education. The milestones under investigation include crawling, walking, babbling, first, 10th, 50th word, and first multi-word utterances. The data was collected with a specially designed mobile app, in which parents reported their children's development repeatedly. Using this relatively big sample and looking at a wide range of investigated milestones, we present evidence that typical bilingual development follows a trajectory similar to monolingual development. We also evaluate the feasibility and usefulness of online data collection using mobile apps to study early language development.