There are a host of national and international guidelines for private equity and venture capital firms. Links to relevant ESG-related guidelines can be found below.

UN Principles for Responsible Investment (PRI) 
The PRI are six principles that aim to help investors in all asset classes integrate Environmental, Social and Governance issues into investment decision-making and ownership practices. In total, there are about 900 signatories to these principles: asset owners, asset managers and service organisations. 

The UN PRI has a special Private Equity Workstream that publishes documents on responsible investment in private equity, for example:

Invest Europe Industry Standards & Guide to ESG Due Diligence
The European trade association Invest Europe has developed several initiatives and guidelines to put sustainability on the agenda. 

American Investment Council Guidelines for Responsible Investing
Members of the American Investment Council have adopted a set of comprehensive responsible investment guidelines that they apply before investing in companies and continue to apply during the period of ownership. The directives cover environment, health, safety, labour, governance and social issues.

The UN Global Compact Principles
The UN Global Compact is for companies committed to sustainability and corporate responsibility. It provides both a policy platform and a practical framework. It is based on 10 universally-accepted principles in the areas of human rights, labour, environment and anti-corruption and offers a wide range of management tools, resources and cutting-edge programmes.

The Walker Guidelines 
The Walker Guidelines are a set of guidelines for disclosure and transparency in (UK) private equity, which were published in 2007 and apply to portfolio companies, the private equity industry and trade associations (BVCA).