What Is an ATS? Understanding Alternative Trading Systems

Equity trading no longer occurs in one place. While the traditional exchange-only centralized model is now a multi-venue marketplace, liquidity is dispersed across exchanges, dark pools, electronic communication networks (ECNs), and more than 30 Alternative Trading Systems (ATS). Now, trades travel through a network of interconnected (but often siloed) platforms, meaning that the market is harder than ever to see, predict, and assess in real time.

That fragmentation has tangible effects. Firms will confront increased operational complexity, stemming from venue-specific rules, custom APIs, and the challenge of inconsistent data formats and increasing infrastructure costs. Then again, ATSs have created other possibilities, such as differentiated liquidity, anonymized block trading, and faster execution (if firms are able to bear the complexity).

In this blog, we provide the answer to a fundamental question: what is an ATS? How are alternative trading systems operating, under what Regulation ATS (Reg ATS) rules as these venues, and why do ATSs have a main role to play in modern market structure, from liquidity generation to openness and data complexity?

How Do ATSs Work, and What Types Exist?

To appreciate how contemporary equity markets operate, we must learn how these Alternative Trading Systems (ATSs) function. Given the proliferation of trading over dozens of venues, ATSs were formed as an integral component for how liquidity is accessed, matched, and traded, particularly on channels other than the traditional exchanges.

What is an ATS?

At a higher level, ATS stands for alternative trading venue and it is the platform from which buyers and sellers are matched outside traditional public markets like the New York Stock Exchange and Nasdaq. Although ATSs do not list securities or exist as a public exchange, they are increasingly important in negotiating prices and finding possible liquidity.

ATSs function under Regulation ATS (Reg ATS) and are required to be registered as broker-dealers and present Form ATS-N, making clear how the venue functions, who can use it, and how orders are handled. This positions ATSs between fully lit exchanges and fully bilateral trading in practice.

What are the Main Types of ATSs?

There are two primary categories of ATSs: Electronic Communication Networks (ECNs) and dark pools. The key difference between them comes down to how much information is visible before and after a trade occurs.

ECNs

• High-speed, automated matching engines
• Can offer displayed or non-displayed liquidity
• Commonly used for continuous trading and latency-sensitive strategies
• Increase price competition, but add venue-level complexity
• Often considered “lit” venues due to higher pre-trade transparency

Dark Pools

• Provide anonymity for institutional and block trading
• Reduce signaling risk and adverse price movement
• Offer midpoint and other non-displayed liquidity
• Heavily used for large trades where discretion matters
• Often described as “semi-lit” due to limited pre-trade visibility

Why This Distinction Matters

ATS types behave differently, publish different data, and fulfill different trading purposes. For companies that have offices across dozens of different venues, they aren’t mere academic differences — they influence the interpretation of liquidity, the measurement of execution quality, and the normalization and downstream analysis of market data.

What Is Regulation ATS (Reg ATS), and Why Does It Matter Now?

Alternative Trading Systems, a subtype of ATS, have been a less regulated market since their inception. Although ATSs have for a long time had to register with the U.S. Securities and Exchange Commission (SEC) and abide by Regulation ATS (Reg ATS), the architecture has held to nearly the same basic structure for most of the last 20 years despite substantial upward momentum for off-exchange trading volumes.

At least in part, that stability was based on how ATSs were used. The bulk of ATS activity was conducted outside the core exchange-moderated trading day and at long time windows sometimes extending up to (more often) 23 hours or more, five days per week, rather than as part of the core trades. For an extended period, those hours remained at the margin of the market and limited regulatory urgency, further cementing the view that ATSs were secondary venues rather than central infrastructure.

Why Regulatory Attention Is Increasing Now

With trading activity increasingly driving to near-continuous markets, regulators and market operators are re-examining longstanding assumptions about equities infrastructure. As growth in overnight and extended-hours trading and the emergence of new off-exchange trading venues like 24X, the SIP Operating Committee has submitted a proposal to the SEC to extend SIP operating hours to better mirror how and when trading actually takes place.

That proposal represents a larger recognition of the fact that off-exchange venues, ATSs included, are no longer edge cases. They have a central role in how U.S. equities trade, price discovery, and data dissemination go forward. It is a big change with important consequences.

More regulatory attention means that:

• Increasing scrutiny over how ATS data is reported, consolidated, and consumed
• Higher compliance complexity for broker-dealers interacting with multiple ATSs
• More pressure on institutional desks to demonstrate transparency, consistency, and best execution across fragmented venues

To put it another way, Reg ATS is simply no longer a static box to check. It is becoming an evolving structure that is more in tune with how the market truly trades in today’s world. And as monitoring changes with longer trading days and an increase in ATS participation, proper, regular, and trustworthy ATS data even becomes more important.

How do ATSs Contribute to Market Fragmentation?

Market fragmentation did not happen overnight. As demand for 23/5 and near-continuous trading has been increasing, liquidity has moved away from traditional exchange hours, when ATSs were the preferred venues to facilitate access, and where they were ultimately able to operate.

Today, around half of U.S. equity trading volume occurs off-exchange, indicating a consistent flow of liquidity into alternative exchanges that allow extended and overnight sessions. This phenomenon has been powered in significant part by international participation, primarily by APAC and EMEA investors who want to access U.S. equities within their local trading hours.

ATSs are at the center of this pivot. The Cboe analysis of FINRA data revealed that ATSs make up about 25 percent of all trade volumes off the exchange. Over 30 active ATSs currently operate in the U.S., many of them now enable extended or overnight trading in order to take on global, cross-session liquidity.

Fragmentation Is No Longer Just About Venues — It’s About Time

Liquidity, therefore, now splinters in time and among venues. Trading is increasingly flowing perpetually from Europe to the United States and further east into Asia, generating intersecting sessions where price discovery, liquidity creation, and enforcement of the terms take place outside the protections and norms of ordinary trading hours.

That gap has spread, and institutions are having to reconsider where and when they trade. Broker-dealers and non-bank liquidity providers are already engaged in these extended markets, and retail investors continue to demand access to them. Major exchanges have admitted to this, with many indicating that round-the-clock trading is no longer a hypothetical future condition, but a reality of contemporary market structure.

The problem with that is visibility. ATSs produce their data in proprietary formats and have their activity reported at different rates, especially outside of core trading time. Public disclosures like Form ATS-N give useful information, but their architecture isn’t conducive to analysing in real time dozens of venues that operate almost 24 hours a day.

Accordingly, the NBBO is no longer adequate. It represents only a partial, or partial picture of the market with no representation of non-displayed ATS liquidity, midpoint activity or even behavior specific to venues, which increasingly drives outcomes in execution. For firms to get a grip of true liquidity dynamics in a 23/5 environment, they require up-to-date and historical ATS data normalized across venues and time zones.

How Can Firms Enable Market Transparency and Best Execution?

Stable, normalized ATS data is an important part of retaining market transparency and an ideal execution strategy. Traders require a single, real-time representation of liquidity across exchanges and across over 30 ATSs to know where valuable liquidity sits.

With no normalization and harmonization, firms can’t spot venue-specific patterns, fill quality or toxicity. That demands wide exposure to an array of information, including:

• Normalized, low-latency market data from all lit and unlit venues
• Deep-book data to see true liquidity beyond NBBO
• Historical ATS behavior (fill rates, midpoint activity, execution latency, toxicity)
• Transparency data from Form ATS-N (operational details, participant types, segmentation)
• Unified analytics that merge real-time and historical signals

How Can Firms Gain Reliable Access to this ATS Data?

No single firm can meet these challenges alone, as we discussed at Exegy’s NYC Client Summit. As data volumes, costs, and performance expectations pile up, firms are increasingly reassessing whether working to build and maintain this infrastructure internally still makes sense.

As the universe of ATSs continues to grow with many working in extended or overnight sessions, this will not only need engineering effort, it will also need operational support, normalization, and continuous regular updates at venues as these evolve. For many companies, that reality has redirected the build-versus-buy conversation toward strategic partnerships.

This is the shift Axiom and other such solutions aim to respond to. Providing normalized, consolidated market data across exchanges and ATSs, Axiom mitigates the engineering and operational challenges of connecting disparate venues. Standardized symbology, correct timestamps, enriched metadata, and consistent formats offer firms a full and reliable snapshot of U.S. equity liquidity —without the headache and work of accumulating dozens of custom feed handlers.

In a world like the one this article describes, in which markets never sleep and liquidity divides further, leveraging trusted technology partners isn’t only about efficiency. It’s also increasingly how firms keep up with change.

Navigating Market Complexity with a Partner

The ATS ecosystem is also a true opportunity, but also more complicated. As liquidity continues to fragment against additional venues and expanded trading hours, it is hard to get a singular, accurate view of U.S. equity markets.

The reality is that most companies can’t handle such complexity with siloed, internally managed data infrastructure. ATS data (of high quality and normalized) is now core to transparency, execution-quality analysis, compliance, and venue evaluation.

That’s where partnership matters. With Axiom, Exegy’s market data-as-a-service offering, firms gain reliable, normalized access to ATS and exchange data without the burden of onboarding and maintaining dozens of raw feeds. Axiom currently provides access to Blue Ocean ATS with Bruce OTS and Moon ATS in development, helping firms keep pace as new venues come online.

As market structure continues to evolve and new trading venues emerge, working with a provider purpose-built for normalization and scale positions firms to adapt with confidence.