Automated Trading on MT5: What Expert Advisors Do

The short answer

An expert advisor — an EA — is a program that runs inside the MetaTrader 5 terminal and can analyse prices and place orders without you. MetaQuotes, which publishes the platform, describes them this way:

"Expert Advisors are mechanical trading systems that allow complete automation of analytical and trading activities for the efficient operation in the financial markets."

Everything below is taken from that documentation, because mechanism is the part you can establish as fact. What no documentation can tell you is whether automating a given strategy is a good idea, and we are not going to imply otherwise.

How one is built

An EA is not a setting you switch on. It is software, and it exists because someone wrote it.

The platform ships with its own language for the purpose:

"The trading platform has its own built-in language for programming trading strategies MetaQuotes Language 5"

and its own editor for working in it:

"MetaEditor is an integrated MQL5 development environment. It is a component of the trading platform. MetaEditor allows you to create, edit, compile and debug source code written in MQL5."

The step people often miss is compilation. Source code is not what runs:

"compile it by pressing F7 to receive an executable file. The executable file has an extension EX5 and can be run in the trading platform."

So the thing executing on your account is a compiled binary. If you obtained an EX5 without its source, you cannot read what it does, and neither can anyone else you ask.

There is also a route that needs no programming, and leaving it out would misrepresent the platform:

"With the MQL5 Wizard a trader without programming skills can easily create Expert Advisors. You only need to select trading signals for an Expert Advisor, as well as money management and trailing stop algorithms."

That lowers the barrier to producing an EA. It does not lower the barrier to producing a good one — the selections still encode a strategy, and the strategy is still the hard part.

How one is started, and the limits on that

An EA does not run globally. It runs on a chart:

"To start an Expert Advisor, attach it to a chart. The easiest way is to double-click on an Expert Advisor in the Navigator window or drag'n'drop it to a chart."

And the chart holds exactly one:

"Only one Expert Advisor can run on one chart. If you start another Expert Advisor on the same chart, the first one is removed."

That second sentence is worth reading twice if you intend to run several strategies. The replacement is silent — the platform does not ask whether you meant it. Each EA needs its own chart.

Separately, trading can be switched off while the EA itself keeps running:

"If the icon is [Auto Trading disabled], the Expert Advisor is not allowed to perform trading operations."

An EA that appears to be running and is placing no trades is a normal state, not necessarily a broken one. It is the first thing to check before concluding a strategy has stopped working.

What the documentation does not say

Being precise about the gaps matters more here than filling them.

The sentence everyone repeats — that an EA only acts while the terminal is open and connected — is not stated on the pages we read. It is very likely true, and it follows from how a desktop program works, but we could not find MetaQuotes saying it, so we are not quoting them as if they had. If that fact matters to your setup, confirm it against your own broker's documentation rather than taking it from an article.

More importantly, nothing in the documentation speaks to whether automation improves results. The docs describe a mechanism for executing a strategy faster and more consistently than a person would. If the strategy has no edge, the mechanism will express that with great consistency. Where the apparent edge usually comes from — and why it so often evaporates — is the subject of why good backtests fail live, which is the companion piece to this one.

Where an EA sits among the tools

An EA is automation of execution. It is not, in itself, a prediction engine, and it belongs to a different category from the model-based products discussed in the wider tool landscape. The difference between a strategy written as explicit rules and one fitted to data is set out in rules versus learned parameters.

Nothing here is a recommendation to automate anything; see the risk disclosure.