1. Core Legal Answer & Context: The relationship between international law and domestic law is a fundamental question in legal theory, with two primary theoretical approaches: Monism and Dualism.

Theories of Relationship:

• Monism: This theory views international law and domestic law as part of a single, unified legal system. In a pure monist system, international law is automatically incorporated into domestic law and takes precedence over conflicting domestic law. If there's a conflict, international law prevails. Hans Kelsen is a prominent proponent, suggesting a hierarchical order with international law at the apex.
• Dualism: This theory posits that international law and domestic law are two separate and distinct legal systems, operating in different spheres. For international law to have effect within a domestic legal system, it must be transformed or incorporated into domestic law through specific legislative acts (e.g., an Act of Parliament). Without such an act, international law has no direct effect domestically. Heinrich Triepel and Dionisio Anzilotti are key dualist thinkers.
• Harmonization/Coordination: Many modern approaches recognize that neither pure monism nor pure dualism fully captures the complex reality. Instead, they emphasize a need for coordination and harmonization between the two systems, acknowledging their distinct but interacting roles. States often adopt a mixed approach, incorporating some international norms directly while requiring transformation for others.

Application of International Law by National Courts in India: India generally follows a dualist approach with a strong inclination towards transformation for treaties and a more monist approach for customary international law, provided it is not inconsistent with domestic law.

• Treaties: For international treaties to become part of Indian domestic law, they generally require legislative action by the Parliament under Article 253 of the Constitution of India. This means a treaty obligation does not automatically become enforceable in Indian courts unless Parliament enacts a law to give it effect. However, courts may interpret domestic statutes in a manner consistent with India's international obligations where possible.
• Customary International Law (CIL): Indian courts tend to apply CIL more directly, provided it is not contrary to existing Indian statutes or the Constitution. The judiciary often presumes that the Indian legislature does not intend to violate international law. CIL can be used to fill gaps in domestic law or to interpret ambiguous statutes.
• Judicial Activism: The Indian Supreme Court, particularly in human rights cases, has shown a tendency to refer to and apply international human rights instruments even without specific enabling legislation, especially when there is a vacuum in domestic law and the international norm is not inconsistent with domestic law. This reflects a "soft monist" or "harmonization" approach in certain areas.

2. Relevant Statutes and Sections:

• Constitution of India, Article 51 (promotes international peace and security, respect for international law and treaty obligations).
• Constitution of India, Article 253 (Parliament has power to make any law for implementing any treaty, agreement or convention or any decision made at any international conference, association or other body).

3. Important Landmark Cases:

• Vishaka v. State of Rajasthan (1997): The Supreme Court of India explicitly referred to the Convention on the Elimination of All Forms of Discrimination Against Women (CEDAW) and formulated guidelines to prevent sexual harassment at the workplace, stating that international conventions and norms are to be read into domestic law in the absence of specific legislation, provided they are not inconsistent with the Constitution or existing laws.
• Gramophone Company of India Ltd. v. Birendra Bahadur Pandey (1984): The Supreme Court held that international law is not a part of Indian law unless it has been specifically incorporated by legislation. However, it also stated that in case of ambiguity, courts should interpret domestic law in a manner consistent with international law.

4. Clear Conclusion: The relationship between international and domestic law is complex, with India largely adopting a dualist approach for treaties requiring transformation, but showing a more accommodating stance towards customary international law and international human rights norms. This is particularly evident through judicial interpretation and activism, ensuring consistency with its international obligations where domestic law is silent or ambiguous, thereby reflecting a pragmatic blend of dualist and monist principles.