A large amount of data that is generated today is unstructured, which requires processing to generate insights. Some examples of unstructured data are news articles, posts on social media, and search history. The process of analyzing natural language and making sense out of it falls under the field of Natural Language Processing (NLP). Sentiment analysis is a common NLP task, which involves classifying texts or parts of texts into a pre-defined sentiment. You will use the Natural Language Toolkit (NLTK), a commonly used NLP library in Python, to analyze textual data. In this tutorial, you will prepare a dataset of sample tweets from the NLTK package for NLP with different data cleaning methods. Once the dataset is ready for processing, you will train a model on pre-classified tweets and use the model to classify the sample tweets into negative and positives sentiments. This article assumes that you are familiar with the basics of Python (see our How To Code in Python 3 series), primarily the use of data structures, classes, and methods. The tutorial assumes that you have no background in NLP and nltk, although some knowledge on it is an added advantage.