What causes ADD/ADHD in children?

At the moment there is no definitive answer to this question! There are many studies looking at genetic causes, as well as environmental causes (such as post-natal brain injury, exposure to tobacco smoke, exposure to toxic elements) but no conclusive data has yet to be determined. There is also interest in how food affects ADD/ADHD symptoms but there is again no conclusive evidence. What we do know is that ADD/ADHD is not a result of parenting and is not the child being lazy and unmotivated.

Adding to what has been said above (by CHI.L.D) we also know that making the diagnosis is a difficult process.  Sometimes young people (and adults) present with symptoms like ADD (for example poor concentration) that actually have a different cause, such as depression or auditory processing problems.  Getting the diagnosis right is critical.

As the other experts have indicated, it is a complex disorder with genetic and environmental indications, and can have multiple influences including diet. Diagnosis is arbitrary and there is a large overlap between different developmental disorders - in many cases it can be more productive to address the symptoms. There is good evidence for the role of diet in exacerbating ADHD symptoms - see http://fedup.com.au/factsheets/symptom-factsheets/adhd-and-diet for some background and helpful information. Some children may also benefit from omega-3 fatty acids, zinc, and/or pycnogenol (anti-oxidant grape seed extract) if suboptimal levels of these nutrients, which are critical for brain function, are contributing to symptoms.

It is difficult to have a problem where no answers can be found. In such circumstances it is important to be aware of what it is not caused by just as much and in this regard there are 2 things to put your mind at ease: it has nothing to do with vaccination (that came from a doctor in the uk who was being paid by lawyers wanting to sue drug companies, so he made stuff up to suit his employers needs) and number 2, it is not your fault. A comprehensive review of hearing, sleep, and vision are good starting points for ruling out other diagnoses or contributing pathologies. And forget about Dr Google- seek professional advice not random blog posts from unidentifiable sources

For most people, ADHD is hereditary: we now know that ADHD is about as heritable as height. Although a number of genes have been identified (e.g. the dopamine transporter gene, DAT1, on chromosome 5), we haven't yet identified all of them nor do we know how they work together to "cause" ADHD. Early life events (such as smoking or drinking during pregnancy, very premature birth, toxic lead levels, or even brain injury) can also contribute to about 20% of all ADHD. Interestingly some researchers now think that many of these early life events may be linked to parental ADHD, and so genetics may still contribute to some of that 20%. 

We do know, without question, that ADHD is not caused by "bad" parenting or "bad" teachers, nor is it caused by sugar, TV or computer games. Poor diet can make symptoms worse although it will not cause ADHD. There is some evidence to suggest food intolerances may cause ADHD-like symptoms in some people. If verified, this research simply means those few people had similar symptoms to ADHD; it does not mean that they had ADHD, nor that it was caused by their diet. 

This is an important point: There are a lot of things that can look like ADHD: dietary problems, sleep issues, anxiety, depression, bipolar disorder, OCD, thyroid problems, lead poisoning, and more. And, of course, ADHD can look like any of those things. This is why clinicians always recommend a thorough assessment by an experienced professional before diagnosing ADHD.

In short, for most people ADHD is genetic. However the genetics are complex and environmental factors — such as diet, parenting, exercise, schooling and use of electronic devices — can all impact how ADHD is expressed, experienced and managed. A thorough assessment considers all these, along with alternate explanations for the symptoms, in order to make a diagnosis and recommend helpful strategies for managing the condition.

Adding to what the clinical health professionals have written, this (slightly technical but free-access) review summarises what is known about the complex interplay between genetic and environmental risk factors for ADHD;

http://www.ncbi.nlm.nih.gov/pubmed/22963644