Graph Neural Networks, Explained: How Message Passing Turns Relationships Into Predictions

Graph Neural Networks (GNNs) are specialized neural models designed for interconnected data, utilizing a 'message passing' mechanism to learn from relationships between entities. This approach enables GNNs to uncover patterns and make predictions in complex networks that traditional neural networks cannot directly handle.

David Katzman

November 12, 2026 · 5 min read

Graph Neural Networks (GNNs) are a class of neural models built specifically for data that lives as a web of interconnected entities — social networks, financial transactions, molecular structures — and they work by passing information between connected nodes until each node's representation reflects its surrounding network. That mechanism, called message passing, is what lets GNNs find patterns in relationships that traditional networks like CNNs and feed-forward models cannot directly see, because those architectures assume data arrives in a fixed grid or vector rather than a tangle of connections.

What are Graph Neural Networks?

Most interesting phenomena break down into entities, their relationships, and their interactions, and graphs are the mathematical abstraction that captures this structure: nodes represent entities and edges represent the connections between them. NVIDIA's research documentation, authored by Matthias Fey and Ivaylo Bahtchevanov, frames graphs this way, and GNNs are the neural network family designed to learn directly from that structure.

The need is real and broad. A survey paper by Jie Zhou and colleagues on arXiv notes that modeling physics systems, learning molecular fingerprints, predicting protein interfaces, and classifying diseases all demand models that learn from graph inputs. In these domains, the connections between data points carry as much signal as the data points themselves — a molecule's behavior depends on which atoms bond to which, and a transaction's risk depends on which accounts and devices it touches.

As Rick Merritt reported on the NVIDIA blog, GNNs express these elements mathematically so machine learning algorithms can make useful predictions at the level of nodes, edges, or entire graphs. That flexibility is the core promise: the same architecture can classify a single node, score a relationship, or characterize a whole graph.

The Message Passing Mechanism: How GNNs Learn

Message passing is the engine at the heart of GNNs. IBM's explainer describes it simply: nodes exchange information with their neighboring nodes and aggregate that information to update their own representations — similar to nodes in a graph exchanging messages or signals. Each node begins with an initial embedding that acts as its input features, and from there the process repeats in rounds.

Each round has three conceptual steps. First, a node collects information from its neighbors. Second, it aggregates that information — combining the incoming signals into a single summary. Third, it updates its own representation based on that summary. After one round, a node knows about its immediate neighbors; after two rounds, information from neighbors-of-neighbors has flowed in; after multiple rounds, as the NVIDIA documentation puts it, a node representation encodes progressively larger neighborhoods.

This is where the power emerges. Because GNNs propagate information along edges, topology and multi-hop patterns become model features. A node with few attributes of its own can still be highly informative when its neighbors are informative — a person with a sparse profile becomes predictable from the company they keep. The Distill introduction to GNNs by Benjamin Sanchez-Lengeling, Emily Reif, Adam Pearce, and Alexander B. Wiltschko walks through exactly this construction, building a modern GNN piece by piece from these propagation steps.

One caveat worth knowing: not every architecture labeled a GNN performs full message passing. IBM notes that graph convolutional networks, while simple and suitable for large-scale graphs, do not support edge features and in that sense lack the message-passing notion, restricting them to cases where all required information lives in the nodes. Message passing neural networks, by contrast, allow representations of edges — which matters when the relationships themselves carry the signal.

GNNs vs. Traditional Neural Networks: A Fundamental Difference

The clearest way to understand GNNs is to contrast them with the architectures most readers already know. Convolutional neural networks assume grid-like data such as images, where spatial locality matters and fixed-size convolutions slide across pixels. Standard feed-forward networks (MLPs) process tabular or sequential data as fixed-size input vectors. In both cases, relational information must be explicitly encoded into the input features before the model ever sees it — the network has no inherent mechanism for reasoning about connections.

GNNs invert that assumption. As CyberSaint's glossary entry by Maahnoor Siddiqui observes, unlike traditional neural networks that process data in a linear or grid-based format, GNNs are purpose-built to capture complex, interconnected relationships among data points. Stanford's Jure Leskovec put it more bluntly in a talk quoted by NVIDIA: GNNs are catching fire because of their flexibility to model complex relationships, "something traditional neural networks cannot do."

GNNs vs. Traditional Neural Networks: A Comparative Overview

The table below maps each architecture's core operation to the data format it assumes, so you can identify which model fits a given problem.

Architectural comparison of GNNs, CNNs, and traditional feed-forward networks
Dimension	Graph Neural Networks (GNNs)	Convolutional Neural Networks (CNNs)	Traditional Neural Networks (MLPs)
Data Structure	Graph-structured data: nodes (entities) and edges (relationships); applies to data naturally represented as graphs	Grid-like data such as images, where spatial locality is important	Tabular or sequential data requiring fixed-size input vectors; graph data often needs pre-processing to fit
Core Operation	Message passing: nodes iteratively aggregate neighbor information and update their representations over multiple rounds to capture broader network context	Convolutions on fixed-size inputs, enabling local feature extraction	Matrix multiplications on fixed-size inputs, with no inherent mechanism for direct relational inference
Typical Use Cases	Social network analysis, fraud detection, drug discovery, recommender systems — illustrative examples, not an exhaustive list	Image recognition and pattern recognition in structured grid data	Learning complex functions from feature vectors
Key Advantage	Captures complex relational patterns and dependencies in interconnected data, learning rich representations of nodes and edges — fundamental to their design	Optimized strengths for pattern recognition in grid-structured data	Optimized strengths for specific data formats and feature-based tasks

Architectural comparison of GNNs, CNNs, and traditional feed-forward networks

Dimension

Graph Neural Networks (GNNs)

Convolutional Neural Networks (CNNs)

Traditional Neural Networks (MLPs)

Data Structure

Graph-structured data: nodes (entities) and edges (relationships); applies to data naturally represented as graphs

Grid-like data such as images, where spatial locality is important

Tabular or sequential data requiring fixed-size input vectors; graph data often needs pre-processing to fit

Core Operation

Message passing: nodes iteratively aggregate neighbor information and update their representations over multiple rounds to capture broader network context

Convolutions on fixed-size inputs, enabling local feature extraction

Matrix multiplications on fixed-size inputs, with no inherent mechanism for direct relational inference

Typical Use Cases

Social network analysis, fraud detection, drug discovery, recommender systems — illustrative examples, not an exhaustive list

Image recognition and pattern recognition in structured grid data

Learning complex functions from feature vectors

Key Advantage

Captures complex relational patterns and dependencies in interconnected data, learning rich representations of nodes and edges — fundamental to their design

Optimized strengths for pattern recognition in grid-structured data

Optimized strengths for specific data formats and feature-based tasks

The comparison yields a clean distinction: GNNs are the only architecture in the table whose core operation directly consumes relationships, rather than requiring them to be flattened into input features beforehand.

Real-World Applications of Graph Neural Networks

The applications follow directly from the mechanism. In fraud detection, financial networks connect accounts, transactions, devices, and merchants — and a fraud pattern can look perfectly ordinary in an isolated transaction while becoming visible across the two- or three-hop network of related accounts and events, as the NVIDIA documentation explains. Merritt's reporting adds that fraud detection systems specifically use edge embeddings to find suspicious transactions, and Amazon reported in 2017 on using GNNs to detect fraud, later rolling out a public GNN service in 2020; Amazon Search also employs GNNs to detect malicious sellers, buyers, and products.

Recommender systems model user-item interactions as a bipartite graph, with purchases, views, or ratings as edges. The learned representations capture behavioral similarity through the graph structure itself, extending beyond collaborative filtering on a flat matrix. A concrete form of this, per the NVIDIA blog, is node embedding used to match customers with products.

Drug discovery exploits the graph-level view: molecules are naturally graphs of atoms and bonds, and drug discovery models compare entire graphs of molecules to find out how they react to each other. The same relational reasoning supports the graph-adjacent tasks the arXiv survey highlights, such as learning molecular fingerprints and predicting protein interfaces. The Distill authors point to a widening practical footprint — antibacterial discovery, physics simulations, fake news detection, traffic prediction, and recommendation systems — and researchers are exploring further use in computer graphics, cybersecurity, genomics, and materials science, including using transportation maps as graphs to improve arrival-time predictions.

These are illustrative examples rather than an exhaustive list, but they share one signature: in every case, the connections between data points are where the predictive signal lives.

When to Choose a Graph Neural Network

Opt for a Graph Neural Network when the underlying data inherently possesses a relational structure, and understanding these relationships is critical for the task. The measurable indicator is the presence of interconnected entities where the connections themselves carry significant information or influence outcomes — accounts linked by shared devices, users linked by purchases, atoms linked by bonds. When relationships are incidental to the problem, a CNN or MLP optimized for grids or feature vectors will usually serve better; when a node's meaning depends on its neighbors, message passing is the mechanism that turns that dependence into a prediction.