We study the design and decoding of high-rate space-time codes in two contexts. The first part of the paper examines the multiple-access channel (MAC) where users employ space-time block codes (STBC). The problem is formulated in the context of an inter-symbol interference (ISI) channel which occurs for transmission over frequency-selective channels. We show that a diversity order of 2Mτ (v + 1) is achievable at full transmission rate for each user, when we have Mτ receive antennas, channel memory of v and an optimal multiuser maximum-likelihood (ML) decoder is used. In the second part, we examine high-rate space-time codes that have a high-diversity code embedded within them. This can also be viewed as unequal error protection codes designed for unequal diversity order, which is the metric suitable for fading channels.