There are three aspects to fats:
• Dietary fats - the fats we eat,
• Body fat - which are of two basic kinds:
a. the fats that are an integral part of our cells, our brains, our organs
b. the excess body fat that is unwanted and which many people want to remove from their body.
Barry Sears puts it succinctly: "... fat doesn't make you fat. Increased insulin levels do that." (Page 81). Rob Faigin adds: “Ultimately, it does not matter how much fat you eat, it matters how much fat you store” (Page 57).
This page focuses on the fats we eat and the fats that are a vital part of us. 
Fats are one of the three macronutrients, the other two being carbohydrates and proteins. Loren Cordain's research indicates a Palaeolithic macronutrient profile, based on available calories, of 28-47% fat, 22-40% carbohydrates and 19-35% protein - over time, not at each meal (Dr Sears' misleading simplification).
Fats are an essential part of our diet - essential in significant quantities. Popular nutritionists often refer to good fats and bad fats but just where the boundary lies between the good and the bad depends on prejudice, ignorance, fashion and tradition (a polite word which can include commercial vested interest and obstinacy in the medical professions) as much as it does on science.
In fact a balanced fat intake which replicates the types and quantities of fats consumed by our ancestors in the Pleistocene is what we should be aiming for today - with certain qualifications reflecting the ways we have degraded the Pleistocene qualities of our environment through pollution, overcrowding, industrial processes, blinkered technological fixes and depletion of food-producing soils.
The fats we eat fall into three categories based on their molecular structure, categories which also reflect, generally, their nutritional role: saturated fats, monounsaturated fats and polyunsaturated fats. Most natural foods with a significant fat component have fatty acids from each category, but with a different fatty acid profile within each category.
Saturated fats are most prominent in eggs and red meat and are often described as 'bad' by people who prefer black-and-white certainty to the complexity of reality. Evfit stresses the importance of a Pleistocene environment producing healthy food for active people. In this environment, the amounts of saturated fats in the diet would be not only tolerable, they would be health-giving. What is more, the lipid profile of wild animals - whether they be salmon, rabbits, poultry or cattle - is healthier as food for humans than the flesh mass-produced by modern agriculture.
Saturated fats include butter, lard and the predominant fats in cheese and fatty meat. 'Saturated' fats are generally solid at room temperature and less prone than the PUFAs to attack by free radicals - that is, they are less inclined to become rancid.
Monounsaturated fats (MUFAs) are most prominent in food from plants. They are the predominant fats in the Mediterranean diet in which olive oil is a main component. Avocadoes, macadamias and almonds are also good sources of MUFAs. Monounsaturated fats are generally liquid at room temperature and turn cloudy if refrigerated.
Polyunsaturated fats (PUFAs) come from both plant and animal sources. In the Palaeolithic, most PUFAs would have come from animal sources but sophisticated manufacturing technologies introduced in the 1940s and 1950s meant that oils could be produced readily from seed such as sunflower, soy and canola. The story of how trans fats manufactured from polyunsaturated vegetable oils replaced saturated fats in manufactured foods with the aid of political clout, and an uncritical medical profession is told in 'The Oiling of America'.
Trans fats are also described as hydrogenated and partially hydrogenated fats; they are manufactured products.  They were not present during human evolution and so could play no part in the shaping of Homo sapiens. They are manufactured from omega-6 fatty acids into a product with no nutritional virtue at all and in fact they disrupt the human metabolism. Although hazardous to human health, they have commercial merit: they turn liquid fats into more (commercially/industrially/financially) attractive hard fats. Indeed, 'a study published in the American Journal of Public Health [Willett and Ascherio, 1994] concluded that consumption of trans fats by Americans was responsible for more than 30,000 deaths from heart disease' (Cordain page 51).
The omega-3 and omega-6 fats are found within the PUFAs. Both omega-3 and omega-6 are essential to human health  and cannot be manufactured within the human body. With the shift from Palaeolithic hunter-gathering to Neolithic cropping and domestication of animals, the omega-3 / omega-6 ratio slipped from its Palaeolithic norm of around 1:2 or even 1:1 to today's western average of about 1:10 and American average of 1:20 . 'An excess of omega-6 fats increases your risk of heart disease and certain forms of cancer; it also aggravates inflammatory and auto-immune diseases.'  Although cereal grains have a low proportion of fats among their three macronutrients, they are generally consumed in relatively large quantities and so their omega-3 / omega-6 ratio becomes significant for health outcomes.
Sources of omega-3 and omega-6 fatty acids - Omega-3s are produced in blue-green algae and in green leafy plants and grasses, as well as in the meat of the animals (including fish, eggs and dairy products) that eat this vegetation. Humans should therefore eat fresh greens, fish and grass-fed meat. Grain-fed meat (including eggs and some farmed fish) has a much greater proportion of omega-6. Humans and other animals that consume diets dominated by grains are prone to develop gastointestinal-based disturbances. 
The bottom line on dietary fats
1. For more on human body fat see the page on body shape; for an illustration of obese 21st century males see this page.
2. One class of trans fats - vaccenic acid - exists naturally in minute quantities in some meat and dairy products.
3. Body by Science, page 192
4. If your diet resembles that of a hunter-gatherer, you'll be ingesting omega-6 fatty acids and omega-3 fatty acids at a ratio of 1:1 or 1:2. With this health-promoting ratio, a large component of your cell walls will comprise omega-3 fatty acids, and because these fatty acids' molecules are elongated and flexible, the cell walls will be fully expanded, placing all of the hormonal receptors on the exterior of the cell facing outward toward the environment, where they can appropriately interact with circulating hormones. If the ratio of omega-6 to omega-3 gets to four to one, a breakdown occurs in hormonal function, which can impair the loss of unwanted body fat. The typical Western diet has a ratio of twenty to one meaning the majority of the cell wall comprises fatty acids that are short, brittle and less flexible. So the cell wall will be thinner and, thus, somewhat involuted. Therefore a multitude of the hormonal receptors necessary for fat mobilization are likewise going to be involutes - facing inward on the cell wall where they cannot interact with the environment. (Body by Science, page 192) Back to text
5. The Paleo Diet, page 50. McGuff and Little add that omega-3 fatty acid sources are the backbone and precursors to the series 3 prostaglandins, which have an anti-inflammatory effect. The omega-6 fatty acid sources, as might be expected, are precursors to the series 6 prostaglandins, which are markedly inflammatory. Not only is the cell wall negatively impacted by over-consumtion of omega-6 fatty acids, but also the body's inflammatory state is disrupted. It's common for people with this kind of imbalance to develop irritable bowel syndrome or gluten sensitivity. Even cattle that consume grain-based diets typically develop gastro-intestinal disturbances; there are many more problems with E. coli contamination in grain-fed beed in contrast to grass-fed beef. Omega-3 fatty acids maintain the body's cell walls with their hormone receptors placed to best interact with their environment. (page 193) Back to text
6. Body by Science, page 193 Back to text
7. Binford tabulates data from 390 hunter-gatherer ethnographies including their population density, their proportional dependence on hunting, gathering and fishing food sources and where they can be classed in terms of being settled or nomadic. Back to text
Lewis Binford - Constructing Frames of Reference 
Loren Cordain - The Paleo Diet
Rob Faigin – Natural Hormonal Enhancement
Doug McGuff and John Little - Body by Science (esp. Chapter 9)
Barry Sears - The Omega Rx Zone
Mary Enig and Sally Fallon - The Oiling of America