A farmer selling wheat and your local water company are both selling something. But one of them can put the price up tomorrow and lose nobody, and the other cannot. That difference has a name: market power. Market structure is simply the study of where a firm sits between those two extremes.
📘 What you need to know
Market structure describes the shape of a market: how many firms are in it, what they sell, and how hard it is for a new firm to join.
There are four structures to learn: perfect competition, monopolistic competition, oligopoly and monopoly.
Market power is a firm’s ability to push its price above the cost of making one more unit, and get away with it.
A firm with no market power is a price taker. A firm with market power is a price maker.
More market power usually means higher prices, less output and more market failure.
We judge market power using market share, the concentration ratio and the size of barriers to entry.
What actually decides a market structure
Four things. Learn these four and you can place any market on the spectrum without memorising a list.
How many firms, and how big they are. One firm with 100% of sales behaves nothing like 5,000 firms with 0.02% each.
The product. Is it homogeneous (identical, so buyers do not care whose they buy) or differentiated (buyers can tell the brands apart and have favourites)?
Barriers to entry and exit. Can a new firm walk in tomorrow, or does it need a licence, a pipeline network and $2bn of start-up money?
Information. Do buyers know every price in the market, or can a seller quietly charge more because nobody is checking?
The four names are just four points on one line. Nothing changes suddenly at the borders — a market with six firms is simply further right than a market with sixty.
The four structures side by side
This is the table to have in your head before you draw anything. Everything later in the topic is an explanation of one column.
Feature
Perfect competition
Monopolistic competition
Oligopoly
Monopoly
Number of firms
Very many, all tiny
Many small firms
A few large firms
One
The product
Identical
Similar but branded
Identical or branded
Unique, no close substitute
Barriers to entry
None
Low
High
Very high
Power over price
Price taker
A little price setting
Price maker, but watches rivals
Price maker
Profit in the long run
Normal only
Normal only
Abnormal can last
Abnormal can last
Real example
Wheat, currency trading
Cafes, barbers, nail bars
Supermarkets, mobile networks
Local water supply
Notice the two rows that do the real work: barriers to entry and long-run profit. They are linked. Profit only survives in the long run if something is stopping new firms from coming in and competing it away.
Market power, and why it is a problem
Market power is not about being big for the sake of it. It is about what a firm can do to price and output because rivals cannot punish it.
The test for market power
Can the firm raise price above marginal cost — and keep most of its customers?
Follow the chain. A firm with market power raises price above MC. Because price is above the cost of the last unit, some consumers who valued the good more than it cost to make it walk away. That output is never produced. Resources are not going where society values them most, so the market is allocatively inefficient. On top of that, with no rival snapping at its heels, the firm has less reason to cut waste, so average costs drift above their minimum — productive inefficiency. Both of those are market failure, which is why governments care.
Watch the wording. “Abuse of market power” is a source of market failure in exactly the same way pollution is. Nobody is breaking the law — the market is simply producing the wrong quantity at the wrong price on its own.
How we measure market power
You cannot measure “power” directly, so economists use signs of it.
Market share — one firm’s sales as a percentage of total market sales. Many regulators start asking questions above about 25%.
Concentration ratio — the combined share of the largest few firms. A high five-firm ratio means a handful of firms hold the market between them.
Barriers to entry — the highest barriers give the most durable power, because they protect the profit from being competed away.
🤔 Why “market power” and “market competition” are not the same thing
Two firms can both be huge and still have little market power, if they are locked in a brutal price war and customers switch instantly. Power comes from what customers and rivals let a firm get away with, not from the firm’s size on its own. Always ask: if this firm put its price up 10%, what would happen?
Worked examples
WORKED EXAMPLE 1
Identify the market structure in each case, giving one reason. [3]
(a) Around 40 independent bakeries in a city, each with its own recipes and regulars. (b) Four firms supply 92% of the country’s mobile phone network, and a new network needs a government licence plus billions in masts. (c) A single firm owns the only rail line into a town.
(a) Monopolistic competition
Many small firms, easy to open a bakery, but each loaf is branded and slightly different → a little price setting power.
(b) Oligopoly
A few firms hold nearly all the market and the barriers to entry are very high (licence + huge sunk costs).
(c) MonopolyOne seller, no substitute for that route → a pure price maker.
One structure + one reason = the mark. Do not write a paragraph.
WORKED EXAMPLE 2
A firm sells $27m of a product in a market worth $150m in total. Calculate its market share and comment on its market power. [3]
Step 1: Put the numbers into the formulamarket share = 27 ÷ 150 × 100Step 2: Work it out= 0.18 × 100Market share = 18%Step 3: Comment
18% is well below the 25% level regulators usually watch, so the firm probably has some but limited market power — the rest of the market can still undercut it.
Always finish with a sentence of judgement. The number alone rarely gets the last mark.
💡 Exam tip
When a question says “explain the market structure”, give number of firms + product + barriers. Three quick points, done.
Say price taker or price maker early. Examiners look for it and it drives the whole diagram.
Barriers to entry are the reason profits do or do not survive. Mention them whenever the long run comes up.
Real examples earn credit. Keep two or three ready that you can actually name.
For evaluation, remember market power has upsides too — profits can fund research and economies of scale. Never write it off completely.
Market power is a source of market failure. Linking it back to that phrase pulls the whole topic together.
⚠ Common mix-up
Monopolistic competition is not monopoly. The names look alike; the markets are opposites. Monopolistic competition has many firms and easy entry.
Big firm does not automatically mean monopoly. A large firm in a market full of rivals may have very little power over price.
Homogeneous does not mean cheap. It means buyers see the products as identical, so nobody will pay extra for one brand.
A monopoly is not “one firm in the world”. It is one seller in a defined market, which is often local — the only pharmacy in a village counts.
High market share is a clue, not proof. If entry is easy, a firm with 60% today can lose it next year.
Do not confuse barriers to entry with barriers to exit. Sunk costs stop firms leaving, which is a separate problem.
Up next: Profit Maximisation and the Rational Producer — the one rule every firm in every structure follows, and the diagram you will draw over and over.
Want this explained one-to-one?
Book a free session with an experienced IB Economics tutor and get your trickiest topics made simple.
