Northeast Luzon is the area encompassing Cagayan Valley, the Northern Sierra Madre Mountain Range and the eastern part of the Cordillera Mountains. Cagayan River is the biggest river system of the Philippines. Originating in the highlands of the Sierra Madre in the East, the Cordillera in the West and the Caraballo Mountains in the South, it flows North through the broad Cagayan Valley ending in the Babuyan Channel.

The Sierra Madre Mountain Range is situated on the eastern side of Cagayan Valley and extends south from the extreme tip of Northeast Luzon to Central Luzon. The highest peaks of the Sierra Madre are about 2,000 m. The Cordillera is situated on the western side of Cagayan Valley and covers the entire central part of northern Luzon. The highest peaks here are nearly 3,000 m. The Caraballo Mountains in the South form the natural barrier between the Central Luzon plains (in which Manila is situated) and the Cagayan Valley. Numerous rivers from the Cordillera and Sierra Madre Mountains feed the Cagayan River.

The low-lying, flat floodplains of the Cagayan Valley are intensively cultivated. The main crops are corn, irrigated rice and tobacco. Towards the Sierra Madre Mountain Range, the landscape becomes hilly. The main crops here are corn, upland (rain fed) rice and banana. The Sierra Madre is steep and still mostly covered with forest. The portion of the Sierra Madre within Isabela Province has been declared a protected area: the Northern Sierra Madre Natural Park.

The climate in Cagayan Valley is characterized by a relatively dry season from January till May and a wet season from June to December. Rainfall is erratic, with most millimeters brought in by typhoons, averaging 1750 mm. per year in the valley but much higher in the mountains (no data available). Average temperatures range from 24°C in January till 29°C in June. About 2,5 million people inhabit the Cagayan Valley.

The Cagayan River is the lifeline of the valley. Nearly all towns are situated along the banks of the river. The fertile floodplains provide prime agricultural lands. Fisheries used to be very important but unsustainable fishing has all but depleted fish stocks in the river itself. Tributaries of the Cagayan River are still important fishing grounds but are fast being depleted as well.