This paper (i) reviews temperature/development rate relationships in plants and poikilothermic invertebrates, (ii) argues that the relationship is often linear over much of the range up to the thermal optimum (To) and provides a possible mechanism, (iii) provides evidence of a trade-off between the base temperature (Tb) and the thermal constant (DD) that enables each species to adapt to its thermal environment, and (iv) indicates some of the practical and ecological implications. Where a linear relationship has been characterised it is possible to estimate the base temperature for development (Tb, expressed in °C) and the thermal constant for development (DD, the reciprocal of the temperature coefficient (a), expressed in degree [°C] days accumulated above Tb). A possible basis for the linear relationship between rate and temperature is proposed based on the Arrhenius and Sharpe-Schoolfield equations involving activation enthalpy and progressive inactivation of the reactant molecules at both low and high temperatures. Knowledge of Tb and DD enables rates of development of organisms/processes to be calculated and compared at any given temperature between Tb and To. An analysis of published results for differentiation processes (differentiation = a change of state) in species of insects, Collembola, spiders, nematodes and plants showed that Tb tended to vary with the temperature of the niche to which the organism is adapted, and that there was a trade-off between Tb and DD. Tropical species had higher values of Tb than temperate and DD decreased as Tb increased (and vice versa). This conferred a competitive advantage on each species in the thermal environment to which it was adapted. The decrease in DD tended to be relatively greater than the increase in Tb, further favouring a high Tb in tropical species. A mechanism for the trade-off is suggested whereby DD and Tb were shown to be correlated (P < 0.01) with the activation enthalpy (HA) of an assumed, rate-limiting enzyme. Thermal time can also be applied to processes involving growth (= an increase in dry weight) when the DD requirement for development to maturity is the sum of the requirements for differentiation and growth. Rates of both differentiation and growth can vary greatly between species, depending upon the niche they inhabit, and the implications of such differences for resource requirements are considered. In insects and nematodes, but not in annual plants, development is usually coupled to growth. Consequently, when resources are inadequate, mature size in these animals varies less than in plants. Thermal time is shown to provide insight into the life strategies of species within their communities and to have practical implications.