The discreteness of biological components, whether they be proteins in
a cell or individuals in an ecological population, has important
consequences for the dynamics of biological systems. This concept is 
quite natural for statistical physicists, but not widely recognised
in the life sciences. I will discuss some examples relevant to cell
biology, cancer progression, and embryo development, in which
stochastic process theory along with agent-based computer simulations 
have helped provide new insights and understanding.