complex biological systems involve multiple space and time scales. to get an integrated understanding of these systems involves multiscale modeling, computation and analysis. in this talk, i will discuss two such examples in cell biology and illustrate how to use multiscale methods to explain experimental data. the first example is on chemotaxis of bacterial populations. i will present recent progress on embedding information of single cell dynamics into models of cell population dynamics. i will clarify the scope of validity of the well-known keller-segel chemotaxis equation and discuss alternative models when it breaks down. the second example is on the axonal cytoskeleton dynamics in health and disease. i will present a stochastic multiscale model that gave the first mechanistic explanation for the cytoskeleton segregation phenomena observed in many neurodegenerative diseases.