Reading the ingredient tree of any product, either a skin care product or (perhaps more importantly) a food product is a skill many ignore. For example, if you buy bacon, do you ever check the actualy meat content? Some supermarket bacon is only 87% from an actual pig - the rest is mainly water and ... what?
That ingredient tree is not only useful for identifying what is actually in what you buy as much as noting where what is supposed to do the work, appears on that ingredient list. Ingredients are listed according to how much of the formula they make up, from highest to lowest until reaching 1%. Anything lower than 1% does not need to be listed. Do not, however, think that, just because an ingredient is low down the list means a product is no good; some ingredients are active at low doses Allantoin, for example or Coenzyme Q10 (aka Ubiquinone).
The usual first ingredient of many products is water. In a day or night cream, this can be as much as 65-70% of the total content of the formula (water content is especially higher when you forage in the lower end of the market. Don't worry too much, though - you and I are 60% water). The quality of the water used, therefore, is vital. Some natural skincare brands opt to replace the purified water with, say flower water, something that at first glance seems a like good alternative but in some instances is nothing more than water used for washing plants before processing; such not only has no practical benefit but also a high level of impurities. Some manufacturers try to avoid water altogether and use only oil-based products, which can produce other issues. Oils alone, it is suggested, are not able to hydrate skin sufficiently (water helps to feed the hydro-lipid film of the skin) and oils can - and do - go rancid. In some cases, formulas containing oils in their mixture begin to separate - not too much of an issue as long as the product is well shaken before use.
There are a significant number of writers who will tell you all sorts of horror stories about some of those ingredients used, with varying degrees of accuracy. Overall, most ingredients used in skin care ingredients (certainly those sold within the EU) are basically, safe. If you are not sure, then rely on a factual source if you want to know about any ingredient. Perhaps the most reliable source is the European Chemicals Agency (actually ECHA - an agency of the European Union) and you can look up any product which you wish to know more about. The ECHA was instrumental is seeing an end to ingredients such as Hydroquinone (which is harmful if swallowed, causes serious eye damage, is suspected of causing genetic defects, is suspected of causing cancer and may cause an allergic skin reaction) - in skin lightening products. A startling comparison is that whereas the ECHA has about 1,300 banned ingredients, the USA has banned only about 30.
Remember that in all cases, to much of even a "good" thing can cause issues. It is often not so much a matter of what is in the product you are buying, as to how much of that product is actually in what you are buying. Sodium Lauryl Sulfate, for example (a foaming agent) is quoted by some as making skin too clean. If you are going to bathe in pure SLS, it probably will do a lot of damage though where a case may be made for not accepting the ingredient at all, it is necessary to remember that some not-quite-as-desirable ingredients are neccessary in order to make any given product perform.
Parabens are another ingredient that has a mixed reception. Again, the EU states that one or several Parabens can be present in a given product. The maximum total concentration allowed in such consumer products is 8 g of parabens per kg of cosmetic product, with no single paraben having a higher concentration than 4 g/kg. According to the ECHA: "The group of chemicals known as parabens make up an important part of preservatives which can be used in cosmetics. In addition to Propylparaben and Butylparaben, other parabens like Methylparaben and Ethylparaben, are also safe, as repeatedly confirmed by the Scientific Committee on Consumer Safety (SCCS). They are also some of the most efficient preservatives avaliable."
Retinol is another ingredient that comes under fire from time to time. Retinol includes the active substances acitretin, adapalene, alitretinoin, bexarotene, isotretinoin, tazarotene and tretinoin. These substances (note: NOT all used in skin care!) are taken by mouth or applied as creams or gels to treat several conditions mainly affecting the skin, including severe acne and psoriasis. Certainly, oral retinoids can harm the unborn child and must not be used during pregnancy. Topical retinoids (those applied to the skin) must also not be used during pregnancy, and by women planning to have a baby. And Yes, you must always use a good quality sunscreen.
In conclusion, read the ingredient tree but remember - It is not that a given product is there - it is about how much of it is there and what it is there for.