An Expert Advisor is a program written in MQL that runs inside MetaTrader and can open, modify and close trades on its own. It is the format underneath nearly every retail forex bot on the market.
An Expert Advisor (EA) is an automated trading program for MetaTrader 4 or MetaTrader 5, written in MQL4 or MQL5. It attaches to a chart, receives every price tick, and can place, modify and close orders without human input. EAs run inside the terminal, so the terminal must be open and connected, which is why serious users run them on a VPS.
| Runs | Can trade? | Typical use | |
|---|---|---|---|
| Expert Advisor | Continuously, on every tick | Yes | Automated strategies |
| Indicator | Continuously | No | Drawing and calculation only |
| Script | Once, then exits | Yes | One-off actions: close all, set stops |
The distinction matters for safety. An indicator cannot touch your account no matter what it contains. An EA has full trading rights the moment you allow it to run, which is why the source of an EA matters far more than the source of an indicator.
An EA has three entry points. OnInit() runs once when it attaches to a chart. OnTick() runs on every incoming price update and holds the trading logic. OnDeinit() runs when it is removed or the terminal closes.
Two consequences follow directly:
OnTick() never fires. An EA cannot manage a position while it is not receiving prices. Which is exactly why a laptop that sleeps is a real risk rather than an inconvenience.MT5 adds OnTimer() for scheduled execution and OnTradeTransaction() for order events, which makes some designs cleaner than they can be on MT4.
Four limits are structural rather than a matter of build quality, and understanding them is most of what separates people who use automation well from people who lose money to it.
It cannot know what it has not been shown. A rule set encodes past relationships. When the regime changes (a trending market turns to chop, a correlation that held for two years breaks) the system keeps applying yesterday's logic with full confidence and no awareness that anything has changed.
Backtests overstate almost everything. A backtest with clean fills, fixed spread and no slippage is not a simulation of trading, it is a simulation of arithmetic. Add realistic spread, variable execution and the requeues that happen in fast markets, and a strategy that looked excellent frequently becomes marginal.
Optimisation finds coincidences. Tune enough parameters against enough history and you will find a setting that fits it perfectly. That setting describes the noise in that particular sample, not a property of the market, and it stops working the moment it meets data it was not fitted to. This is curve-fitting, and it is the single most common reason a purchased system fails.
Recovery logic hides risk instead of removing it. Martingale and grid systems produce beautiful equity curves for months, because doubling into a losing position converts many small losses into rare enormous ones. The curve is not evidence of an edge; it is evidence that the loss has not arrived yet.
MarketPro is building an Expert Advisor for MetaTrader 4 and MetaTrader 5. It is not available to download yet, and this page will say so until it is.
What is open today is the waiting list. Install the app, open the EA tab, and join it; everyone on the list is emailed when the builds go live. There is no charge and no card involved in joining.
When it does ship, three things will be true about it by design:
Meanwhile the thing that is live is the signal feed: vetted trade ideas with entry, stop and three targets that you place yourself. That is the manual equivalent of what the EA will automate, and it is available today with one free signal a day.
Seven steps. The last one is the one people skip and the one that matters.
In MetaTrader, File → Open Data Folder. This is the real installation path, not wherever the terminal appears to be installed.
Put the .ex4 or .ex5 (or the .mq4/.mq5 source) into MQL4/Experts or MQL5/Experts. Any custom indicators it needs go in the matching Indicators folder.
Restart the terminal, then right-click Expert Advisors in the Navigator and choose Refresh. The EA should appear in the list.
Click the AutoTrading button in the toolbar. Nothing trades until this is on, and it is the fastest global kill switch you have.
Drag it onto the chart for the instrument and timeframe it was designed for. In the dialog, tick "Allow algorithmic trading" and review every input before confirming.
A smiling face in the top-right corner of the chart means it is running. A sad face or a cross means AutoTrading is off or the EA is not permitted.
Always. An EA has full trading rights on the account. Run it on demo long enough to see a losing streak before it ever sees real money.
The MarketPro Expert Advisor for MetaTrader 4 and MetaTrader 5 is not available to download yet. The in-app waiting list is open, and everyone on it is emailed the moment the builds go live. Vetted signals are available today in the app. Get MarketPro free and join the EA waiting list from the EA tab.
The MarketPro Expert Advisor is software you install and run yourself on your own MetaTrader terminal. MarketPro does not trade on your behalf, does not manage your account, and exercises no discretion over your funds. It is supplied as-is with no performance guarantee, and availability is limited by region.
The MarketPro EA is in development. The vetted signal feed is live now, with one free signal every day.
Not investment advice. Past performance is not indicative of future results.