Multiple turbocharger control

A turbocharger system comprises: a gas input for receiving exhaust gases from an associated engine; a first turbocharger, arranged to be driven by exhaust gases received at the gas input; a second turbocharger, arranged to be driven by exhaust gases passing through the first turbocharger or exhaust gases received at the gas input and being coupled to an electrical generator operative to provide electrical power; and a controller. A diversion mechanism may be configured to affect the flow of exhaust gases between the gas input and the first turbocharger. The controller may be configured to adjust the operation of the electrical generator on the basis of the energy of the received exhaust gases, to maximise the proportion of the energy of the received exhaust gases provided to the first turbocharger or on the basis of a parameter that is a function of the energy of the received exhaust gases.