In fluid dynamics, the Basset–Boussinesq–Oseen equation (BBO equation) describes the motion of – and forces on – a small particle in unsteady flow at low Reynolds numbers. The equation is named after Joseph Valentin Boussinesq, Alfred Barnard Basset and Carl Wilhelm Oseen.
One formulation of the BBO equation is the one given by Zhu & Fan (1998, pp. 18–27), for a spherical particle of diameter , position and mean density moving with particle velocity – in a fluid of density , dynamic viscosity and with ambient (undisturbed local) flow velocity 
This is Newton's second law, with in the left-hand side the particle's rate of change of linear momentum, and on the right-hand side the forces acting on the particle. The terms on the right-hand side are respectively due to the:
- Stokes' drag,
- pressure gradient, with the gradient operator,
- added mass,
- Basset force and
- other forces on the particle, such as due to gravity, etc.
The particle Reynolds number
has to be small, , for the BBO equation to give an adequate representation of the forces on the particle.
with the material derivative of Note that in the Navier–Stokes equations is the fluid velocity field, while in the BBO equation is the undisturbed fluid velocity at the particle position: