Swimming micro-organisms in fluid environments are ubiquitous and diverse. Such micro-organisms frequently undergo movement patterns which can be modelled as random walks. The presence of external cues such as light, chemical gradients, or gravity can modify individual-level behaviour resulting in the random walk being biased towards a preferred direction. In still fluid, at the population level, this can mathematically described as a diffusion process with drift. In many natural environments, swimming occurs in fluid environments undergoing motion. The fluid flow will interact with the swimming behaviour in several ways and alter the spatial distribution of a population, leading to such phenomena as bioconvection and gyrotactic focussing. Here I present the derivation of a population-level advection-diffusion model for gyrotactic algae which is based on extensive experimental observations of individual swimming cells in still fluid. I will then present theoretical predictions and experimental data describing the behaviour of gyrotactic micro-organisms in 3D flow. Finally I will discuss the effect of flow on the population-level spatial distribution.