Being overweight or significantly underweight can both be unhealthy. A BMI between 18.5 and 25 is considered ideal. A BMI of 30 or more is obesity and when it reaches this level it can lead to health problems.
60% of the adult population is overweight or obese.
Our weight is usually a simple balance between taking in enough energy to deal with our day to day requirements and most of us take too much energy in the form of food and expend too little energy in the form of exercise. Busy modern lifestyles mitigate against easily getting the balance right. Also there is still a lot which is unknown though - genetic, metabolic and psychological - and some people find it incredibly hard to lose weight, though many of us simply don't tackle it in the right way. There are also some medical conditions which are associated with obesity and more research is uncovering new insights into weight all the time.
Modern foods can be much higher in energy content than is obvious (and "low fat" foods often contain high sugar) and portion size is often problem.
We often eat for comfort or as a displacement reaction or simply miss the fact that we have reached satiety because we are distracted by another activity such as watching TV whilst eating.
The NHS.UK healthy eating pages, "Eat Well" are really excellent.
Check what you are eating: Learn what the food labels mean.
Learn what is the best for you: Consider healthy food swaps.
For health, balanced food diet advice: Eat Well Guide
Eating well on a budget: Healthy eating need not be more expensive and can be surprisingly easy - check these tips
Improve your exercise and fitness: NHS.UK Exercise pages (Look at how much exercise will benefit you - even walking and look at the really useful - and fun - the "Couch to 5K" graduated programme and App)
The BBC Website also has useful diet advice and plans on its "Good Food" pages
There are loads of commercial weight loss programmes if you feel the need for someone to check up on you - though an online App or joining a group. Many people use Weightwatchers or Our Path .