In this paper, I deal with the principle of sustainable development, as containing the principle of proportionality, in the context of the liability of company managers for the tax debts of companies from the imposition of environmental taxes and related sanctions, after a relevant research of court judgments (through electronic legal information databases). The aim is to investigate whether the enhancement of sustainable development is achieved through the mandatory collection of environmental taxes and environmental fines of companies from their managers, as third parties. The first part of the paper refers to the liability of company managers for the tax debts of the latter [1], where the relevant legislative framework of the Greek Tax Procedure Code and the relevant case law are presented [2] and the issue of the liability of company managers for the failure to submit the companies' sustainability reports is highlighted, as well as the issues of their judicial protection (1). Subsequently, the limitation of the rights of company managers by their aforementioned joint and several liability is commented on, especially with regard to the right to property/ownership and the right to effective judicial protection (2). Finally, the possibility of protecting the environment through taxation and tax sanctions is examined, where I propose the term “Tax Environmental Law” with sustainable development [3] as the main axis through fair taxation (3). In conclusion, my proposals de lege ferenda are presented. Ultimately, it is concluded that it is in accordance with the principle of proportionality to impose environmental taxes and related sanctions in order to protect the environment [4], within the framework of the principle of sustainable development. In relation to the aforementioned liability of company managers, it is proposed to interpret the legislative provisions on joint and several liability by applying the principle of proportionality. Thus, the unconditional liability of company managers for the tax debts of companies by their mere capacity is not acceptable, but the "causal connection" of the tax debt and the related liability must be proven by the tax audit bodies, without a reversal of the burden of proof. Otherwise, the joint and several liability of company managers would act as a deterrent to the undertaking of the management of companies, with adverse consequences for the economy and the "market". By applying the principle of proportionality, the restrictions on the individual rights of those managing companies for the tax debts of companies will be limited, to the point where the "hard core" of their individual rights is not affected, while at the same time the public interest objective is achieved. This objective is the enhanced protection of the environment [5], without affecting the economic and business freedom of taxpayers, in order for the "market" to function smoothly with elements of free competition. It is therefore understandable that the goal is to promote economic development, under the condition of environmental protection and it is emphasized that "economic development" and "environment" should operate complementary and balance. This is achieved through the effective operation of the principle of sustainable development.