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Two elements are necessary to form any extended extragalactic radiosource: twin beams emanating from an active nucleus and ambient gas to confine the resulting radio lobes. The dissertation presents a study of the physical processes that produce this phenomenon. Methods are developed to allow evaluation of the controlling physical parameters: the bulk power in the jets; the luminosity of the active nucleus; and the density of the confining gas. Comparisons of these quantities provide new constraints to theoretical models of radiogalaxies. The confining gas densities are estimated from either X-ray or optical observations. Extended X-ray luminosity can be related simply to gas density via standard thermal bremmstrahlung formulae but X-ray data of suitable sensitivity are sparse. Thus, an additional assumption about the relation of the depth of the potential well, estimated from the properties of the host- and associated-galaxies, to the gas density must be made for many sources. Jet powers are evaluated by matching a self-consistent physical model to the properties of the <i>extended</i> radio structure. Lobe energy content is directly measurable from the radio map and the source age - which is required to convert stored energy to time-averaged power - is simply the linear size divided by the rate of source expansion (V). The values of V measured directly by radio spectral-gradient techniques are consistent with the adopted physical model in which V is controlled by the internal lobe <i>pressure</i> and the external gas <i>density</i>; this allows V to be estimated for the majority of sources that lack spectral-gradient studies. Nuclear luminosity is estimated from the luminosity of the narrow emission lines which are shown to be produced by photoionisation from the active nucleus. As the line-emitting regions are far from the nucleus (~ 1 kpc) the values of nuclear luminosity obtained are both time-averaged and independent of the angle of the radiosource to the plane of the sky. A major programme of spectrophotometry with the University of Hawaii 88' provided homogeneous [0III]4959/5007 line luminosities for a large unbiased sample of classical double radiogalaxies. The analysis of the relations between these controlling parameters has revealed a proportionality between jet power and nuclear luminosity. This proves, for the first time, a <i>direct</i> link between the accretion process and the powering of the radio jets that feed extended radiogalaxies. An extension of these arguments to radioquasars, radio-quiet quasars and BL Lac objects indicates that:- a) time-variability and dust obscuration alone control the classification of a classical double radiosource of <i>given intrinsic nuclear luminosity</i> as a quasar or a galaxy, although on average quasars have higher nuclear luminosity. b) BL Lac objects and most core-dominated radioquasars have Doppler-boosted radio cores. c) Radio-quiet quasars are physically distinct from radioquasars. The origin of this dichotomy appears to be intimately tied to the optical properties of the host galaxy.