A downhole assembly includes an apparatus for generating a fluid pressure pulse downhole. A disclosed apparatus for generating a fluid pressure pulse downhole includes an elongate, generally tubular housing defining an internal fluid flow passage and having a housing wall. The apparatus also includes a device for selectively generating a fluid pressure pulse, the device having a cartridge which can be releasably mounted entirely within a space provided in the wall of the tubular housing. The internal fluid flow passage defined by the tubular housing is a primary fluid flow passage. A secondary fluid flow passage has an inlet which communicates with the primary fluid flow passage. The cartridge houses a valve actuable to control fluid flow through the secondary fluid flow passage to selectively generate a fluid pressure pulse. Data relating to a measured downhole parameter or parameters can be transmitted to surface via the pressure pulses.